The 408 cipher, mailed in three parts to the San Francisco Chronicle, San Francisco Examiner and Vallejo Times-Herald on July 31st 1969 had 18 unsolved characters at its foot. However, the Zodiac Killer likely made an error when transferring his original message into the encrypted version, accidentally omitting the word "people" from the section of code mailed to the San Francisco Chronicle: "The best part of it is that when I die I will be reborn in paradice and all the (people) I have killed will become my slaves". Had he transferred the message correctly, the intended version would have had only 12 unsolved characters in which his identity may have resided, such as "I am the Zodiac". After all, he promised us his identity not his name. What are the chances the Zodiac Killer left a similar message at the foot of the 340 cipher?
Professor D.C.B. Marsh, in the October 22nd 1969 San Francisco Examiner newspaper article by Will Stevens, laid down a challenge to the Zodiac Killer to reveal his name. The newspaper stated "Dr Marsh told the Examiner today: "The killer wouldn't dare, as he claimed in letters to the newspapers, to reveal his name in the cipher to established cryptogram experts. He knows, to quote Edgar Allen Poe, that any cipher created by man can be solved by man. Zodiac has not told the truth in his cipher messages to the Examiner, the Chronicle and the Vallejo Times-Herald. Zodiac has not done this, because to tell the complete truth in relation to his name -in cipher code - would lead to his capture. I invite Zodiac to send The American Cryptogram Association a cipher code - however complicated - which will truly and honestly include his name". It was speculated that the Zodiac Killer certainly wasn't going to provide another conventional cryptogram to be easily decoded, so instead created the appearance of a cryptogram with hidden "puzzles" contained within it, such as "paradice" and "slaves" running vertically and horizontally across its mid-section. This was later revealed in the 'Halloween' card, where the Zodiac Killer belatedly informed us the 340 code was "sorry no cipher" and placed "paradice" and "slaves" in a cross formation to point us in the right direction.
When Professor D.C.B. Marsh laid down the challenge to Zodiac, it was initially considered that the message at the foot of the 340 cipher may have teased the American Cryptogram Association with something like "next time I will tell you who the Zodiac is", bearing in mind where the near-pseudonym was placed. Then, five months later he would follow this up with the reveal of his name, which he effectively teased us with by stating "This is the Zodiac speaking. By the way have you cracked the last cipher I sent you. My name is...."
This statement is very telling, when we consider that it was patently obvious that his last cipher hadn't been solved - so why did he add this to the opening of the April 20th 1970 communication? The final section of the 340 cipher may explain why he began this correspondence in the way he did. It may be conceivable that the last section of the 340 cipher promised to reveal his name in the next cipher, hence the blatantly obvious introduction in the April 20th 1970 communication, of "This is the Zodiac speaking. By the way have you cracked the last cipher I sent you. My name is...." We effectively have to bridge the gap between "I am the Zodiac" (if correct) on the 408 cipher to "My name is" on the April 20th letter. The obvious connector would be the promise of a new cipher or code containing the Zodiac Killer's name.
It is notable that the 340 cipher begins with "her" (the last three letters of cipher), however, it is impossible to form the word "cipher" in the format offered. The uniform 17 X 20 pattern of the cipher would be broken by adding "cip" to the beginning. For this section to provide us with the word "cipher" within the existing pattern and the promise of a new cipher containing the Zodiac's name, the 340 cipher has to be flipped - possibly as the 'Halloween' card and '13 Hole' postcard had hinted nearly a year later. Other than the 408 cipher mailings, every other Zodiac correspondence began with the introduction of "This is the Zodiac Speaking". The 340 cipher, when flipped, enables this introduction to begin the 340 character message also.
In the April 20th 1970 communication, the killer wrote "By the way have you cracked the last cipher I sent you. My name is...."
Therefore, I will use the literal words of the Zodiac Killer to complete the last 27 characters of the 340 cipher, obviously changing "last" to "next".
By flipping the 340 cipher vertically we are able to use the clue of "Zodiac" to generate his usual introduction and then form the bridge between cipher one and cipher three with the promise of his name in cipher two, using the words of the Zodiac Killer himself. Whether this analysis has any semblance of truth is another matter entirely.