I may need to work harder and explain myself better to convince you guys that the Halloween card independently indicates a bomb in Vallejo Mill (still a forensic opportunity + low-grade risk from a dud; see alternative derivation and exact location in Z32 discussion).

Have a look at the "sorry no cipher" note, which was in the Halloween envelope. The two text lines are perpendicular to each other, but Jacob and others pointed out that the entire structure is at an angle, conveying some message beyond the text itself. What do you think Zodiac was trying to say?

I think it’s a rather transparent attempt to indicate a windmill, i.e. "MILL".

As I said before, Zodiac’s clues are ambiguous, but an explanation simultaneously resolving multiple questions can be recognized as likely correct. We should also appreciate one clue serving multiple purposes, like the double usage of this "sorry no cipher". Zodiac evidently enjoyed the elegance of compressing multiple clues onto one object: a skeleton emphasizing eyes + orange + indicating "il" + forming an L, a second skeleton wearing the pumpkin + orange eyes + letters in eyes, the inverted "M" in "DOOMED" + "BOOM" + "MILL", the ZF-symbol dots simultaneously indicating three different transposition rules, etc.

So far, we saw in the Halloween envelope and card:
Two hints for "VAL" (letters LAV+L emphasized in Paul Averly, Vale=OK by skeleton);
Three hints for "EJO" (multiple ojo’s emphasized + three separate E->O indications);
Three hints for "MILL" (sorry no cipher, M+il+L, cartoon reference). 

You might dislike some or all of these clues, but I think it is becoming harder to deny that Zodiac was trying to say: "VALLEJO MILL".
The same bomb location indicated – independently! – by Z32, which is why I personally am convinced.

________________

You may dislike the use of Spanish, but look at the unusual word "Vallejo" – how would you encode it in Halloween-theme symbols?… I think it’s quite natural for Zodiac to indicate "vale" by an OK gesture, and "ejo" by an "ojo" word play, whether he was fluent in Spanish or had only the casual familiarity expected from a Californian in 1970. I know very little Spanish, yet the ojo->ejo game jumped at me.

The theory is incomplete, but in my opinion is already sufficient, as Zodiac’s clues are deliberately redundant, and most symbol interpretations are not forced (unlike some conspiracy-like theories, easily identified by consistently forced, associative interpretations, as I warned earlier). A more complete theory would be even more convincing, of course, so you are invited to help.

The piece most obviously missing is the pumpkin, which, knowing Zodiac, should serve at least one additional purpose beyond emphasizing orange (as in tree, or a similar indication for the bomb location). The careful and deliberate positioning of the pumpkin on a skeleton’s pelvis seems important. I found a reference for "valla" as a vulgar slang for a woman of such proportions (definition 3; related to Chavala perhaps?), but I don’t think we can use that as relevant for 1970 California unless someone knows more. If Zodiac used more Spanish here, then completing the theory may require assistance from an actual Spanish speaker.

After digging for several more hours, I have little but not much to report yet.

Zodiac was quite efficient, so our theory should really explain every element in the card. People have wondered and speculated on why he added eight eyes in the tree and four eyes to the right of the tree. That’s easy to explain now: Route 84. We already knew from Z32 that a section of this road near Paradise drive was the target.

"DOOMED" may mean bad luck, "jella" in Spanish. In general, word inversions are in the realm of conspiracy theory, but we know from Z340 that Zodiac was prone to such foolishness. The location spelled backwards is "LLIMOJELLAV", so "jella" matches as a partial clue. This is admittedly still weak, but then so is my Spanish.

By the way, ojo -> ejo is not so different from eye -> eyo. So if you really hate the Spanish angle, we have "Valleyo Mill"; pretty close…

I made no progress with the pumpkin. But I must say that the more I look at the image, the more it seems to me like a reference to an oak gall on a branch. It’s probably just my imagination, but if that’s really what Zodiac meant, then finding the bomb will be very easy.

Edit: Sorry – jella is ‘bad luck’ in Italian; I don’t know about Spanish. I could point out that the tiny mask emphasizes no ears (llimo in Spanish) on the doomed, jella skeleton making a ‘V’ shape with his hands, and LLIMO+JELLA+V is Vallejo Mill spelled backwards. Perhaps there is a legit Spanish version that would render this backward reading plausible, so the theory could also explain at some confidence why Zodiac made the mask tiny, chose the specific word "doomed", and picked the V gesture. But again, the theory itself (the card indicates a bomb in Vallejo Mill, etc.; I found some additional evidence to show if needed) is already supported in my opinion by sufficient strong evidence. (And the location, also pinpointed independently by a self-justifying solution to Z32, should be cautiously searched by the authorities.) What do you think?

Is more evidence really necessary?

If I were a Zodiac trying to convey the bomb location in a Halloween-theme card, I would create a compact symbol to indicate this location. Combine the initials V.M.F of Vallejo Mill, Fremont, like so:

Or more compactly:

Or better:

(Ignore the dots – they have a separate use.)

Let’s think some more like a Zodiac. Symbolizing Vallejo is challenging. But the inside of the card (where 8+4 eyes were tellingly added) already has many "eyo" or "ejo", so let us add a "VAL" to this page, too. First, arrange the skeleton’s hands as a V and the rest of his body as an L. There is an "a" in Halloween, but that’s just below the L. Better to show-off by squeezing an unexpected "a" somewhere between V and L. Take the "g" lying there, and carefully carve it into an "a", like so:

Perhaps we as Zodiacs should give another hint for Fremont. Consider the town seal,

On the right, there is a campfire. On the left, there seems to be a flannelbush, common in California and used in the distant past to produce rope. So how about using, in the back of the card, "by fire" and "by rope" (like in the first page of the comics)? This should draw attention, as not real Zodiac crimes, and better than e.g. "by water" (too revealing for this bomb), "by hands", or "by falling" (as in later pages of the comics)? The rope is admittedly forced, but don’t expect Zodiac to write "death by flower"…

While we’re at it, the word "slaves" between these two phrases already has the necessary "LAV", exactly as emphasized on the envelope ("VAL" in reverse; we know Zodiac liked word inversions). Let us draw attention to these three letters, in "sLAVes", by running one text line perpendicular to the first s, and a second line perpendicular to the e rather than the last s. The resulting structure is asymmetric, sure to draw attention to the "LAV" emphasized in between.

We saw one obvious (i.e., orange) and one very questionable interpretation for the pumpkin (plus a few vulgar and unlikely interpretations I won’t detail). Now, the front of the card (the page with the pumpkin) already conveys "vale" and "ejo" in a rather obvious way. While it would be nice for us Zodiacs to add "mill" on the same page, this must be an obscure hint, otherwise the puzzle would become too simple. For example, I think there is already a good chance that a Fremont child would quickly figure out the meaning of the (rearranged)

So as a Zodiac, I would find a clever way to introduce "mill" on this skeleton. Not any pumpkin would do, but this specific one:

may work – just imagine it rotating. Click the image to see a crude approximation.

Again: I’m not sure if Zodiac planned for all the above, but he was careful to introduce redundancy, the theory explains every single element, and some of the interpretations are strong. And that should be enough.

I think these are interesting interpretations, but it’s difficult to know for sure if they are correct.

He could have just been screwing around when he created the Halloween Card. Or perhaps, it does have some greater significance. He did write of of that crap on there, lol. He put that one skeleton on there, as an example, and positioned it in that weird way. He did it, but it’s hard to know if it means anything, I guess primarily due to it’s ambiguity.

I’m not trying to discourage you from posting your work on here. I do think it’s interesting, and perhaps at some point in the future more information will come to light and it could be useful, or maybe even correct, so it’s good to share it, but I think at this point it’s just impossible to know for sure if it’s right.

Thanks, Irvine. Indeed, each clue is ambiguous individually, but this is so by construction: Zodiac clearly did not want the solution to be easy. Therefore, a strategy of going over all card elements, one by one, and asking if each interpretation is solid and unique, will by definition always fail. We will then have to wait for future information coming in, as you suggest, but can we afford 50 more years?

A much better approach is to adopt the scientific method: search for a simple theory accounting for all the observations. The simpler the theory is, and the more observations it explains, the stronger it becomes. Thus, a robust theory will not critically rely on the interpretation of any handful of elements, nor on any ad-hoc assumption.

So, how fares the theory of the Halloween card conveying the same Vallejo Mill bomb location inferred from Z32? Namely, how many elements does the theory succeed vs. fail to explain? Not only does it account reasonably well for every single detail I see in the card, but moreover, it explains multiple roles assigned by Zodiac to most card elements, which is a high bar to impose. By the way, it is not easy to construct such a compact network of interconnected puzzles, so Zodiac demonstrated much more talent here than in the (standard and sloppy) ciphers.

The theory now explains:
four hints for VAL (Paul Averly, vale=OK by skeleton 1, VaL by skeleton 2, sLAVes);
three hints for EJO (three separate sets of ojo’s emphasized + three different E->O indications);
four hints for MILL (sorry no cipher, M+il+L, pumpkin, cartoon reference);
one hint for Route 84 (numbers of eyes added);
one hint for Paradise drive (PARADICE);
one hint for Vallejo Mill, Fremont (ambiguous, VMF or ZF, symbol);
one hint for a bomb (BOOM);
one hint for a tree (a tree…) with three hints for something orange;
and weaker hints (e.g., Fremont seal), some of which require additional (e.g., linguistic) assumptions.

Such diverse support is inconsistent with randomness and renders the theory very robust: you cannot discredit it by ruling out a handful of elements or their interpretations.

Remember that while focusing on the card is fine here, ultimately we cannot ignore the solution to Z32 independently pinpointing Paradise drive in Vallejo Mill. It is standard practice to evaluate the cipher solution independently, and the Z32 analysis made some assumptions (transpositions, “rad”) to arrive at a self-supporting (unique logical location, good cryptanalysis attributes) solution. But eventually, one should combine these two independent communications from Zodiac to evaluate the validity of Vallejo Mill as the bomb location, and I’m not sure the combined evidence leaves a reasonable doubt.

I agree that we don’t know if Zodiac was screwing around: even after informing us, both in Z32 and independently in the Halloween card, that he planted a bomb in Vallejo Mill, he might have done or hidden something else. Fortunately, this question is solvable: we only need the authorities to take a look. In any case, we don’t want a dud on a busy public road, and if we’re lucky, it’s not too late for new forensic evidence while people still care.

Irvine and others have raised the possibility that Zodiac’s clues may be so convoluted that even the correct solution would not be recognized as such. But I would be very surprised if this applies to the Vallejo Mill bomb, as Zodiac introduced more than sufficient redundancy in his Halloween communication, as I now show.

This communication has four parts: envelope (including “sorry no cipher”), front (Zodiac added a pumpkin), inside (added 12 eyes), and back (wrote “By FiRE” etc.). I claim that each part separately conveys the very same message: “Vallejo Mill” (in addition to a few other hints for the bomb and other Zodiac activities). Demonstrating such a redundancy is much stronger than collecting clues scattered throughout the communication, and offsets much of the unavoidable ambiguity inherent to the interpretation of each individual element. The theory requires one plausible assumption: “ojo” (Spanish) can be used for eye; otherwise, you might get “Valleyo Mill”.

(Here I again ignore the very same bomb location inferred separately from the Z32 cipher solution, which has led to this Halloween card theory in the first place. This cipher solution, robustly obtained if one accepts diagonal transpositions as hinted by the ZF/VMF symbol but fairly uniquely even if one doesn’t, should be incorporated when evaluating the actual likelihood of a bomb on the relevant section of Route 84.)

The images below use a high-resolution reproduction (found here), except where its imperfections are misleading. I shifted elements to avoid confusion with unrelated hints, but was careful not to distort or introduce any new meaning without saying so. Unlike my previous posts, below I indicate but do not fully explain a few details I see: perhaps it would be more convincing if you identify some of them yourself. Also, as Zodiac was admittedly better than me in puzzles, you may well find things I’m still missing.

The front of the card is pretty straightforward: with a small addition, it should be an easy-level puzzle for people familiar with the region, especially if they have some experience with this kind of word puzzles:

giving Vale+ejo+Mill, as explained earlier ("vale"=OK in Spanish + "ojo"=eye but o->e in one eye gives "ejo" + "Mill"). The added rolling of the pumpkin is not as forced as it might seem. Before understanding it, we already knew that the careful placement of the pumpkin was not coincidental: the positioning is very symmetric and precisely aligned with the skeleton’s sternum as if to facilitate rolling. Furthermore, as Soze once commented, similar skeleton cards explicitly said “shake, rattle and roll” (here’s one, not sure if contemporary). So, while hinting at a roll was ingenious, Zodiac may have naturally arrived at the idea when choosing a card.

The inside of the card is less obvious, but still seems solvable as a medium-high difficulty puzzle when mildly rearranged,

giving VaL+ejo+MilL, as I explained earlier ("VaL" in skeleton’s arms and body + "ojo"=eye emphasized but OO->EE indications give "ejo" + letters M+il+L emphasized). Zodiac highlighted the M by inversion + text intersection + "BOOM", so I took the liberty of circling it to render the puzzle easier.

The envelope gives the most obvious VAL: Zodiac just underlined “l Av” and provided indications for text inversion, for example with the inverted N (on the back of the envelope) and M (inside the card); also, thanks to the Z340 solution, we know how much he liked inversions. The redundant L added to Avery stands out, and the inside of the envelope alludes to a wind mill, as I argued above. But I didn’t yet discuss the subtle clue for “eye”, and the one or even two subtle hints for replacing an O with an E, all cleverly hidden on the envelope:

Do you see these additional clues? Click the image for a larger envelope version. All combined, here we have VAL+L+ejo+Mill.

Finally, consider the back of the card. I already showed how LAV was emphasized in SLAVES, and that the inverted N in “KᴎiFE” indicates text inversions (and, along with the vertical “By”, also draws attention to a knife, in an apparent Z32 reference). If you caught the “eye” reference on the envelope, then the same hint should be even more obvious here, plus here you can find a second clever hint for "eye":

Do you see the O->E reference? I see one nice such hint, plus one that seems less intentional/elegant. What about the Mill? If you accept the comics reference, as many here have suggested, then a mill is again obvious: the wheel on the cover of the comics is later explicitly associated with a water mill, depicted in the plot climax. Then we have VAL+Ejo+Mill. You also get additional associations: Canyon pass as the real Canyon road, Lady doom related to “DOOMED”, etc.

If you reject the comics reference, then this is the only part out of four where I can’t explain the mill, but then the Fremont seal association becomes more likely. Regardless, Zodiac arranged the text in a meaningful pattern: it may also indicate a railway track, or the two small alleys parallel to Paradise drive in Fremont, or just Route 84 with the parallel railway to its north and Canyon road/Alameda creek to its south. The initials SP may allude to the Southern Pacific railway system. These alternative associations are neither strong nor impossible, and Zodiac may have meant for none or any number of them; they do not repeat, so cannot be substantiated unless someone finds a powerful argument to the contrary.

In summary, following the theory indicates that “Vallejo Mill” appears separately four times, on each part of the Halloween communication independently, along with additional and consistent bomb references which I mentioned here and earlier: (Route) 84, VMF, PARADiCE (drive), BOOM, etc. Introducing such a redundancy required careful attention and considerable talent from Zodiac. Recognizing the coherent structure renders the individual hints far less ambiguous.

Even ignoring the Z32 solution, the redundancy does not leave much doubt that Zodiac was indicating a (failed) bomb in Vallejo Mill. And we really should not ignore Z32. Plus, all we really need now is sufficient evidence to warrant a search of the relevant section of Route 84. After 50 years, perhaps even a shallow inspection may suffice to locate an irregularity. And who knows if Zodiac really meant what he said and what he actually buried there.

Edited for better wording and images.