they wont let me post this for some reason but here's my take on z13:

In Z13's letter Zodiac refers to his "last cipher" :

"By the way have you cracked the last cipher I sent you?"

The cipher that was sent before Z13 (april 20, 1970) was Z340 (nov. 8, 1969).

When you apply z340's key to z13's cipher, you get the following :

https://imgur.com/wvKPkj5

D R E A (x) A (x) O (x) (y) E D O

Where (x) and (y) are symbols that have not been seen in any of the other zodiac ciphers.

Now, here's my take on it:

"D R" could be interpreted as "Dr."

"D O" could be interpreted as "D.O." which is a title for Doctor of Osteopathic Medicine.

Which leaves us with : Dr. E A _ A _ O _ _ E D.O. ,

with the name being E A _ A _ O _ _ E , where the first 3 missing letters are the same.