Just a minor thought here, I’ll admit up front its not exactly a clean solution, but I thought it a bit interesting. Anyway, I believe there is a "Fk Im Crackproof" mail that is not definitively linked to the Zodiac, nevertheless, it’s possible to construct a "crackproof" related solution from the Z13 using reading rules similar to the 340 Solution.
The original Z13 cipher is (using letters in place of the non-alphabetic symbols)
A E N B C K C M C T N A M
The proposed reading rule is to read it symmetrically, that is, to read it in this order: First, last, second, second to last, third, third to last etc. This gives you the transformed cipher
A M E A N N B T C C K M C
One possible way to assign symbols is:
N = 0, A = R, C = A, E = P, B = F, T = unassigned, M = C, K = K
This yields
R C P R O O F _ A A K C A
Here, we shift it to the right by 3 places. I admit this seems a bit arbitrary, but I surmise there might be another reading rule that gives the same cipher but shifted appropriately. This yields:
K C A R C P R O O F _ A A
Finally, if we use the same logic as the end of the 340, where every other word is reversed, this would give us:
C R A C K P R O O F _ A A
The steps required are a little bit arbitary – so I by no means say it’s solved, but I thought it was interesting that the repeated letters O, C, and R line up accordingly. The unassigned space could presumably be any letter (or any unused letter, if we assume a simple substitution cipher was used) – or it could be intentionally left undecipherable to denote a space.
If the T indeed denotes a space, and the initials "A A" might really stand for Arthur Allen. Obviously its hard to say whether this is the intended solution – But it would be interesting to see whether Arthur Allen was known to use the initials "AA" or "ALA". I understand law enforcement seized a lot of written work by Arthur Allen after he died, so presumably this could be checked.
Reasons why I think this solution is possible despite the complicated reading scheme:
1. We know that the 340 also used a transposition to read it, so it make sense the Z13 might too (even though it is shorter)
2. The solution references "crack proof" which might have been mailed by the Zodiac later (gloating by literally telling us the solution knowing we still wouldn’t get it)
3. The "crack proof" just happens to have crack reversed – which just so happens to be how the ending of the 340 was written, with reversed alternating words
4. The way he says "by the way, have you solved the last cipher I sent you?" immediately prior to giving the cipher might be a hint at these similarities.
5. The leftover letters, again, just happen to be "AA" – the initials for the first and last name of the prime suspect (though I fully recognize many people think he just wanted attention).
It’s never going to be possible to confirm a solution for the Z13 given the length, but I feel like this is a good possibility compared to the hundreds of first name/last names generated as solutions.
Finally, hypothetically, if this was the solution – it’s worth asking how we would actually arrive at such an encryption. Working backwards, the way he would have encrypted the message is shown in the picture below, which actually turns out to look quite logical. Starting from the fourth letter, he writes down every second letter until he has completed one cycle – After that, he writes down all the letters he jumped over in reverse order.
Thanks for reading 
