In my research on the ciphers and letters of the infamous Zodiac I noticed something interesting.
At the end of the letter (see image), sent on November 8, 1969, together with the months written by the zodiac, including August, month which we have no crime of public knowledge practiced by the infamous Z ,written in abbreviated form (with the exception of July).
DES JULY AUG SEPT OCT
I noticed that the length of the letters of these "words" is 17, which is the length of the lines of both Z408 (already Decrypted) and Z340 (not yet deciphered) and parsimoniously in the line above Z32 (not yet deciphered)
As this letter was sent along with the cipher 340 (which is suggestive), I thought this might be a key to 340 (Vigenere who knows?!)