1. Research question

The final 18 cipher symbols of the Zodiac Z408 cipher famously do not decode into meaningful plaintext under the established homophonic substitution solution. One proposed explanation is that at least some of these symbols were not enciphered text at all, but were generated by copying (“pulling down”) fragments of ciphertext from rows immediately above.

The question tested here was:

Does the final line of the Z408 exhibit a degree of vertical copying from previous rows that would be unlikely to occur by chance?

The purpose of the test was not to prove intent, but to determine whether the observed pattern is statistically unusual.

Let's take look at the following discussion with ZGPT, a custom GPT put together by user datadude316 and announced on the ZodiacKiller.com message board, here.

Prior to the following conversation I had been discussing some thoughts about potential solutions to the Z32, but then decided to return to the following contentious proposition:

“Turning back to the question as to how we are to interpret the instructions guiding us on what use to make of the Phillips 66 map and the code we were given in 1970, I find myself unable to accept that there exists any plausible alternative to merely accepting that “This map” together with the code will show you where the bomb is set. Once we later have the postscript clue that the encoded distance is in inches, how else are we to consider using “this map” other than to measure the inches on it?”

Let's see how this initial gambit played out.

variously interpreted by Zodiac researchers to represent a gunsight, to be a symbol merely copied (for no especial reason) from the branding of the Zodiac watch, as well as a number of other less readily accepted conceptualisations. But these start out from how we choose to conceptualise the symbol. What, then, of how The Zodiac himself perceived its 'meaning'?

I think we can get close to the author's conceptualisation through careful study of the two key instances where it is clear The Zodiac was using this symbol for a purpose other than as just his signature.

These occur, firstly, on the 'Death Machine' letter of 9 November 1969, as:

and, secondly, on the Phillips 66 map sent with the Z32 cipher on 26 June 1970:

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.

It is common to encounter the error of supposing that we can simply begin with the raw bearing we have applied to the map, couple this with the stated scale of 6.4 miles to the inch and then just run the math. It is, however, not as simple as this.

The Phillips 66 Map used by The Zodiac is, as we shall find, actually not scaled at anything close to 6.4 miles to the inch, nor is the straight-up direction (Map North) properly aligned to True North. It is for these reasons that, through ignoring these combining deviations, we find ourselves regularly presented with proposed bomb locations that do not match the location that would have been obtained by simply following the instructions and laying off the coordinates on the map, as given.

The starting point, then, must be a detailed analysis aimed at identifying the precise degree of each of these primary deviations. And, to do that, we must start with the map itself.

Topographically we have the sloping hillside with adjacent road, surely frequented by school buses for both day outings as well as regular school runs, all combining to meet the logistics of the detailed mechanism we had drawn for us. Yet, more than just this, there is something oddly compelling about the existence of precisely these features at such an elevated and visible landmark—there for all the inhabitants of San Francisco to see and notice daily.

Were there to have actually been an explosion here, people would have been able to stand and point towards it from wherever they found themselves, with only the minimal of effort. With a telescope or an excellent pair of binoculars, it could even be scoped from the very peak of Mount Diablo itself. Even a search here for an ultimately absent device would have been enough to draw the daily attention of tens of thousands at a time.

The North-East face of the twin-peaked hill—precisely that face where the bomb would have to be placed to catch the early morning Summer sun—is just the very face which looms in the distance from many a vantage point along the majority of Market Street, heading west.

And maybe—just maybe—it is a schematic for these very Peaks that we have encoded in the otherwise mysterious symbol added to the Halloween card.

In any event, if there is any new potential solution I see offered to the Z32 cipher, my first thought is always to check whether it will reach from Mount Diablo along the "eighth radian" to somewhere around the middle of the San Francisco Peninsula, and I am all the less excited whenever it doesn't.

""Mr. Z" was a pseudonym used by columnist Nicholas Kristof in 2002 to describe a suspect.....The columns suggested "Mr. Z" may have been involved in the anthrax attacks or previous hoax letters."

https://www.theatlantic.com/magazine/archive/2010/05/the-wrong-man/308019/

The Z-shaped image is coined from the Bates Letters signature.....it is identical in every way.

1. the first and the twenty-sixth letter must be the same;
2. as, also, the second and the final (thirty-second) letter;
3. and, finally, the same for the sixth and the fourteenth letter.

This is all that we have to work with.

Yet, it is perhaps worthwhile considering the question as to why it was these specific three constraints that the author chose to give to us at the outset, and not any of the alternatives that must have been equally possible.

Why, in particular, does he seem especially keen to direct us to the relationship between the very beginning and end of the cipher?