Dave Oranchak showed us how the Zodiac Killer enciphered his letter E's as he traveled through the 408 cipher, by cycling through Z9W+ONE (O with dot). which all represented the letter E and created a repeating sequence (despite breaking down towards the end). This is shown at 32:47 on his Let's Crack Zodiac episode. If we apply a second decoding to the letter E remaining at the end of the 408 cipher, the letter E would now be replaced by the letter Z, which begins this repeating sequence. The 408 cipher would now be complete with a message and signature (or identity). The Zodiac Killer would be signing off his cipher with the letter Z, partially and fully evident in the April 30th 1967 Bates letters and October 27th 1970 Halloween card. Whether the letter E required a second decoding is unproven, but it's a tantalizing prospect regarding a killer who initially boasted "in this cipher is my idenity" - and a missing line that makes this all possible.
Let's Crack Zodiac (Part One) Let's Crack Zodiac (Part Two)