There’s allot of this stuff going in the center of the cipher, it really peaks there.

I noticed that too.

Just curious, did anyone try to search for similar pivots in Z408? Or any other test ciphers? Just to see if Z340 is really that special in this regard?

I have thought about a possible relation between vigenere and the pivots but does that makes any sense?

Well spotted! However, any text of highly repetitive nature is likely to have a lot of pivots appearing (including diagonal ones). It is very well illustrated here (scroll down to the part about testing how often pivots appear by chance in various books). Vigenere table is just a perfect example of highly repetitive text. So it could be Z repeating himself a lot again, like with "kill/killing" in Z408, but probably not "kill" this time . He could also be quoting something, like that "titwillo titwillo titwillo" line from one of his letters.

I’m suggesting that applying vigenere encoding to a plaintext may increase the chances of pivots to appear. Needs to be tested. That the Zodiac first encoded the plaintext with vigenere and then applied homophonic substitution. I believe vigenere prior to homophonic encoding may cause more diagonal patterns/repeats/stuff to appear.

Note that in doranchak’s image, allot of these patterns are appearing in the direction consistent with typical vigenere.

Just curious, did anyone try to search for similar pivots in Z408? Or any other test ciphers? Just to see if Z340 is really that special in this regard?

They can only appear when they are in the plaintext.

25770 tests found at least 2 pivots (6.15%). – doranchak
For all three pairs of assignments to match, which preserves the matching legs of a pivot, the probability is: 0.5 * 0.5 * 0.5 = 0.125 (1 in 8). – doranchak