So, he needed to fold it to hold the three pages together. Where exactly he delivered it to, at or in the offices cannot be ascertained. Was there an outside letter box to the building or did he slip it under a door, or leave it inside the building somehow?

But if we fold the three 10.5 inch pages twice evenly, we get just over 2.5 inches in height. Done in such a fashion would show the bottom section of page 3 from about 8 inches upwards. The only part showing would be NO ADDRESS and the Zodiac sign, All he had to do was place page 3 at the top, and fold in the middle twice. The NO ADDRESS section would now be where he normally placed the address of the newspaper. He didn’t need to address it to the newspaper or editor anymore, because the Zodiac symbol said it all, especially because it could now be read without opening.

I would want to question the analysis given here as to how the letter is assumed to have been folded, and this based on a close study of a higher-resolution version of the same image linked in the OP. In this linked image, the contrast has been washed out sufficiently so that the distinction between the letter and the board on which it is placed is lost. It is, in fact, not as wide as this image would suggest. Indeed, as the annotated high-res image (below) is intended to demonstrate, the one linked to is furthermore heavily cropped from the bottom only.

More to the point, the high-res image clearly shows the lines of folding, which are almost precisely in the expected thirds for standard mailing.

This image has been modified and annotated from the high-resolution image available here.

To be open about the modifications, I have extended the scale on the left proportionately up to zero and down to 11". I have also moved the image of the paper, together with the board it is on, upwards so that the top of the paper coincides with the zero of the scale.

Measurement of the width of the letter according to the same scale on the left shows that it is 7.25", consistent with the expected Monarch size paper apparently favoured by the Zodiac. (Actually, the width in the image tapers slightly in from 7.25 towards the top, which is probably merely a slight perspective effect owing to the alignment of the camera.) It’s length, as you say, should be 10.5". However, as I have discovered in all similar images that I have come across, being apparently only interested in the text on this letter, the bottom has been cropped off mercilessly. I have therefore reintroduced the full size of the paper through the addition of the red box at the bottom.

Although I have had to reduce the size (Kb) of the image to enable me to post it here, it is still of sufficient resolution that two regions of folding can clearly be identified where I have marked with the horizontal red lines. These, as can be seen, sit almost precisely on the thirds – which for a Monarch sheet of paper fall at 3.5" and 7".

The correctly scaled Monarch envelope has then been added to complete the intended illustration at to how, IMHO, this letter betrays in every way that it has been prepared for inclusion in an envelope for posting.

Furthermore, I would want to argue that, were the intention to not use an envelope (as you suggest), but to let the bottom (now) third of this page to serve as it’s cover, then I would regard it as inconceivable that the writer would not have written here his characteristic "Please rush to the editor." I suggest this is absent, because it is on the (merely) missing envelope.

The FBI files stated “Enclosed herewith are the following documents: Original undated three page letter beginning “Dear Editor this is the Zodiac speaking” ending with “did not get front page coverage”. This letter was sent anonymously to the San Francisco Examiner, received, and turned over to the Vallejo Police Department 8/4/69.” The three July 31st letters are mentioned in the same file document alongside their envelopes. Even though this letter was received on August 4th and turned immediately over to the Vallejo Police Department on the same day, no envelope was mentioned in the FBI files.

Context is, I think, important here in order to come to a plausible rationale for why the envelope accompanying the 04 August 1969 ‘debut’ letter was not sent to the FBI, and this can be achieved easily without having to speculate on the fact of its being non-existent, or even missing. If you read the list of four items sent as enclosures, these clearly group into two distinct categories:

1. the original of the 04 August letter; and
2. three full sets, in photocopy, of the documents relating to the 3-part cipher (408)

I see nothing mysterious in this, once it is considered that the purpose of sending a.) is wholly different from that of b.). As the request states:

The original of items 2, 3, and 4 were examined by local police for latent finger prints. Item 1 has not been examined and it is requested that it be processed for latent prints.

To this end, it is sufficient to suppose that the 04 August envelope in question – having been handled by so many that a fingerprint analysis was likely either considered futile, or routinely undertaken by VPD – was merely omitted because no finger print analysis request was being made on this item. (The letter, however, would have been carefully extracted and its potential latents preserved.) So, there was simply no need to send it.

As to the other items, the request is different. These were being sent for a cryptographic analysis, together with a (presumably handwriting) comparison with “other threatening letters submitted to the FBI laboratory.” As these had been received some time previously, analysed (prints) and copied, it was probably the easiest thing to do send them as a job lot with copies of the envelopes included. I would, however, imagine that they would not want to send evidence to no purpose (to protect the chain), and so the most recent envelope was kept back.

Essentially, though, I think it is generally important, before we suppose that there is any indication, in the fact that there is no mention of its envelope in the FBI files, that the killer chose to hand deliver this letter to the Examiner, that we recall the important maxim that “absence of evidence is not evidence of absence.”

[Edited 15 Oct 2022 to correct broken list markup]

Indeed, as the annotated high-res image (below) is intended to demonstrate, the one linked to is furthermore heavily cropped from the bottom only.

Just a few thoughts here…

I think what we see are Xeroxed copies of the original letters? Not sure.

Tom’s site says this letter was post marked San Francisco, which implies there was an envelope. Perhaps he will join in and opine.

That this one seems to have been cropped adds to my belief that there are parts of these letters that we have never seen.

Shaq,

Very interesting work. It begs the question: if there was in fact an envelope, who has it?