I’m curious to hear from cipher experts about how, precisely, they think 408 was enciphered. To me (a novice) it seems like:

1. He came up with an alphabet of ~54 characters.
2. He multiplied letter frequency percentages (from some table? from memory of some table?) by the number of characters in his alphabet.
3. He assigned to each of the 26 Roman letters (by a wheel? drawing from a hat?) an appropriate number of symbols given the calculations in step 2.
4. He created the text for 408, encoding it as he went along.

Does this seem reasonable? I’m especially interested in understanding how letter frequency information was used in enciphering 408, since methods for determining frequency were more primitive in Zodiac’s time, which no doubt resulted in variability between sources. I’ve looked at around 10 Zodiac-era letter frequency tables and can’t find one that matches the "fingerprint" of 408. None of them, for instance, has "f" as a letter that would be frequent enough to merit two symbols in a 54-symbol alphabet.

The idea here is that if we knew which letter frequency table he used (if any), we’d 1) have a better shot at cracking 340 and 2) have more insight into who he was. Was it from a military publication? Something for cipher enthusiasts? Available only in certain libraries?

Interested to hear your thoughts. I’ve always believed the key to 340 is in what we can squeeze out from 408.