This is not a wholly new idea, but I did want to revisit it and set it down in the relevant thread.
I’m intending to provide a run-through of some thoughts and observations I’ve had that might help us form a plausible interpretation of the otherwise mysterious ‘symbol’ that appears both inside, and on the envelope of, the Halloween card sent to Paul Avery in 1970. To start, then, here’s the base symbol in question, as it appears inside the card:

The line that I am planning to take is one that essentially questions the symbol being mysterious at all. In fact, it should be noted from the outset that I shall not be thinking of it conventionally as, in fact, a symbol of any kind, but rather as a straight-up drawing or sketch of something, together with a number of annotations added for the purpose of conveying certain pieces of information—specifically, aspects pertaining to where we are to look for this ‘something’, and where to search on it, once we get there.
“This isn’t right! It’s not even wrong!”—Wolfgang Pauli (1900–1958)
The following, then, is how I would make the distinction between the two facets of this form—the something sketched, picked out in blue on the left of the figure, and the annotations added to the sketch in blue on the right.

Essentially, having chosen to take this mode of interpretation, it becomes difficult to imagine that the ‘something’ being represented here can be anything other than these two ‘peaks’, as we shall come to regard them, and we can therefore move straight on to the annotations and try to decipher what they are intended to convey in relation to hill pair.
“This isn’t right! It’s not even wrong!”—Wolfgang Pauli (1900–1958)
And this brings us to the question of viewpoint.
I will argue that it is intentionally made clear for us that we are not, in our mind’s eye, to be looking down at a two-dimensional flat figure on a page or screen. The four dots, very ably, convey perspective. They set a pattern from which a square arrangement could be formed, but the top two spots are both closer together and smaller than the ‘nearer’ pair and thus we are looking across such a form. And all this adds weight, I feel, that our viewpoint is one of actually looking across at a pair of hills, or a hill with two peaks, from some distance and also from a (relatively) slightly elevated position.
Further, each of the dots, in and of themselves, has a correspondence with one of the four faces of the double peak (near-left/right, far-left/right), and so we can propose without any serious flight of fancy that the ‘line’ annotation is there specifically to pick out one of these for us—the near face of the right-hand peak, as they appear from our assumed vantage point.

“This isn’t right! It’s not even wrong!”—Wolfgang Pauli (1900–1958)
But what, precisely, is our vantage point? Is there anything that we can learn from the Halloween card as to where we might need to be looking to to see such a twin-hilled feature? Certainly, if we had some notion of where we were viewing from, together with the orientation of the peaks in the figure, then we might be able to satisfactorily deduce an answer to this question.
As a first, and reasonable, thought let us put ourselves atop the peak of Mount Diablo.
We are justified in doing so for several reasons. Firstly, this very peak has been referenced before by The Zodiac as a focal point from which to identify something (a bomb) located somewhere in the San Francisco Bay Area. It seems plausible that he may be using this same origin, or viewpoint, again here. Indeed, it has to be suspected at this point that maybe the Halloween card is merely engaging us in precisely the same hunt that we were led into with the Phillips Map and cipher. Are we looking for his bomb on the side of this hill?
Secondly, as the highest point in the locality, the top of Mount Diablo would be just such a point from which we would be looking somewhat down on any such pair of peaks, no matter where they might be located in the Bay Area region.
Let’s see if we can get some help from what else he added near the ‘symbol’ on the Halloween card.

“This isn’t right! It’s not even wrong!”—Wolfgang Pauli (1900–1958)
On the Phillips 66 Map, which The Zodiac sent us along with his Z32 cipher, he drew for our orientation a compass rose over the peak of Mount Diablo, constructed from his zodiacal symbol. We were then given directions indicating how we were to align our search in relation to this peak, and using this ‘compass’. Suppose, then, we do the same with the identified peak we believe we have been drawn to single out on the Halloween card. We have just such a compass already presented to us on the card. All we have to do is move it along to the left, so:

“This isn’t right! It’s not even wrong!”—Wolfgang Pauli (1900–1958)
Which gives us, on a slight head tilt to the right, the orientation:

“This isn’t right! It’s not even wrong!”—Wolfgang Pauli (1900–1958)
Our vantage point, then…
We are taking it as given to us that we are viewing this hill pair from the top of Mount Diablo, thus looking somewhat down on it, and that to our view at this location the orientation of the pair towards our right is to the North. And this, of course, can only be the case if, in looking in the correct direction we, ourselves, are facing broadly West.
This, then, is the direction we should be looking in.
“This isn’t right! It’s not even wrong!”—Wolfgang Pauli (1900–1958)
And from all this, and being emboldened to tie the message in the Halloween card as one inextricably linked to the prior Z32 cipher, I would like to suggest that we could be looking here:
/div>
“This isn’t right! It’s not even wrong!”—Wolfgang Pauli (1900–1958)
Hi @shaqmeister! I’ve never personally interpreted this symbol, but I’d like to share two interesting interpretations that I came across:
1 – Zodiac researcher Morf13 explained here that Fred Harman III, Red Ryder’s inventor son, was the real owner of the cattle brand (here’s the same symbol in the Colorado Brand Book 1948, page 508, n. 29 – source);
https://ostellovolante.com/2020/08/05/zodiac-halloween-riddle-and-italian-cuckoos-game/

2 – From many angles, it appears as a double pyramid and features several secondary peaks. The peak on the left, “North Peak,” stands at 1,084 meters, while the one on the right, “Main Peak,” reaches 1,173 meters. The left peak is 89 meters lower… and sure enough, in the symbol it’s lower than the peak on the right. Meticulous as always, the bastard… But what about that second little leg of the “F” on the right Below is a photo taken from Concord, which lies northeast of Mt. Diablo. As usual, I’ve highlighted the outlines in red…;
https://mdf-alias-mostrodifirenze.blogspot.com/2024/01/il-simbolo-e-la-mappa.html

Hi again, @lendor-77.
Yes, these are two of the more talked-about interpretations of what may be the basis for the ‘Halloween symbol’, if we call it that. I can even recall being on the message boards around the time these, and other, ideas were originally floated and discussed.
For me, though, where both of these fall short in my judgement is that neither interpretation leads us to any understanding of what it is that the addition of this symbol to the card is intended to convey to us. What is it saying?
If the identifiable cattle brand from a Colorado rancher, then … what? What has cattle ranching got to do with anything? Where’s the missing bar that is part of the brand at the bottom, as in appears in Red Ryder? What does the 4-dot annotation say to us, when added to this incomplete cattle brand? For me, then, this line of thinking, although initially mildly compelling, just doesn’t take us anywhere.
The same goes, pretty much, for the double peak of Mount Diablo. What is so important about how it appears from this specific vantage point in Concord? If Mount Diablo has any relevance to us at all in interpreting the symbol on the card, it has to be in connection with his use of this peak as the coordinate origin in locating his bomb on the Phillips 66. But what further information does this figure, interpreted in this way, add to that particular conundrum? Diablo is the starting point of our search for the bomb, not the end point? What purpose does any given view of our origin, here, add to the solution of the problem before us?
So that’s why, having likewise seen in it the line-image of a two-peaked hill range, I have to suppose that it has specific relevance to the end point of our radian, the location where the bomb is set; the place where we would have to go to “dig it up.” And when we start out with this in our minds, then I personally feel that it becomes pertinent now to ask how specifically is it that the annotations—the dots and the lines—are giving us the extra detail that we sorely need, when before we could extract no meaning from them at all.
“This isn’t right! It’s not even wrong!”—Wolfgang Pauli (1900–1958)
With this in mind then—that, with the Halloween card, The Zodiac is once again trying to prompt us with clues as to the location of his bomb (“sorry no cipher [but here’s this instead]”)—my take is that the ‘symbol’ is giving us the location through adding annotation to the outline of a twin-peaked hill, where the annotations—four dots (in perspective) and one line—are intended merely to convey the following:

as from the viewpoint of Mount Diablo.
I can’t speak for how ‘evident’ this idea might appear to others, though, and I acknowledge that, over time, I have probably been seeing the figure in this way to the extent of having ultimately formed an unhelpful bias.
“This isn’t right! It’s not even wrong!”—Wolfgang Pauli (1900–1958)
The right-hand (looking from Mount Diablo), northernmost peak of the pair (Twin Peaks) is Eureka Peak. Both it, and its twin, Noe Peak, do indeed present the correct profile as per The Zodiac’s sketch in the Chronicle letter of 20 April 1970, with the identified face of Eureka being correctly aligned to the morning sun in late summer through to Labor Day.

“This isn’t right! It’s not even wrong!”—Wolfgang Pauli (1900–1958)
Returning to the ‘symbol’ on the Halloween card, I believe it is possible to detect something of its origin in the very card artifacts themselves.
As is now well known, the skeleton figure added to the inside of the card was carefully cut from another Halloween card and added as an insert within this one for whatever specific reason. An original of the card the skeleton came from has been sourced (see here, posted on Reddit), showing us clearly that the eyes of the skeleton are an addition by The Zodiac and that, in its sourced form, it had merely the rectangular opening of precisely the shape that we see in his modified version through which another (cuter) pair of eyes originally peered.
Yet, the eyes remained important (undoubtedly), and so the Zodiac added his own in white on a reddish background.

But if we now focus on these eyes, together with the raised position of the arms and dropped hands, we basically have the majority of the symbol that appears below it. And I, personally, find it hard imagine that the archetype for this symbol was anything other than just this very pose by his added skeleton.
Thus, again to me, I feel I can, with confidence, propose that this figure (symbol), together with its several additions, is not something that has any existence or relevance beyond what it conveys in the context of just this card—essentially, that it is not an insignia of any kind, or a brand, or any such thing having external meaning.
And, furthermore, it now seems pertinent that, for some purpose that we are attempting to decipher, the primary annotations effectively reduce to the lower pair of dots and just the single line.
“This isn’t right! It’s not even wrong!”—Wolfgang Pauli (1900–1958)
Perhaps it is possible then to imagine that, just as the two closer-together dots of the figure symbolise the eyes of the skeleton looking out at us in its prancing pose, so the lower two are our eyes looking back on it, and the single line is the skeleton indicating to us with his left hand, “look here; this one.”
“This isn’t right! It’s not even wrong!”—Wolfgang Pauli (1900–1958)