
It is patently clear by now that if the 13-Symbol cipher contains a name or message, there are going to be thousands of answers that could fit into this short code, that may or may not be correct. The Zodiac Killer almost certainly knew this to be the case after designing two lengthy and complex ciphers of 408 and 340 characters. He must have known that any solution of 13 characters could never be proven to be the correct answer. That is possibly why he didn't create a solution of 13 characters.
After the challenge by D. C. B. Marsh of the American Cryptogram Association on October 22nd 1969, invoking the name of Edgar Allan Poe in his address to Zodiac to reveal his name, he must have hoped that the Zodiac Killer would employ methodology featured by Poe, such as the ones described in his famous essay entitled "A Few Words on Secret Writing". The first three cryptographic methods detailed in Poe's essay were the scytale cipher, which can be used to decode the 340 cipher. The second cryptographic method told of splitting the alphabet in 13 characters of A through M, and N through Z. The Zodiac Killer supplied us with a 13-Symbol cipher on April 20th 1970, beginning with A and ending in M. The third cryptographic method (based on the second method) described two circular pieces of pasteboard, one on top of the other, each containing 26 letters of the alphabet in 26 segments, radiating from the center. One disk was fixed and one was moveable. See here.
After the challenge by D. C. B. Marsh of the American Cryptogram Association on October 22nd 1969, invoking the name of Edgar Allan Poe in his address to Zodiac to reveal his name, he must have hoped that the Zodiac Killer would employ methodology featured by Poe, such as the ones described in his famous essay entitled "A Few Words on Secret Writing". The first three cryptographic methods detailed in Poe's essay were the scytale cipher, which can be used to decode the 340 cipher. The second cryptographic method told of splitting the alphabet in 13 characters of A through M, and N through Z. The Zodiac Killer supplied us with a 13-Symbol cipher on April 20th 1970, beginning with A and ending in M. The third cryptographic method (based on the second method) described two circular pieces of pasteboard, one on top of the other, each containing 26 letters of the alphabet in 26 segments, radiating from the center. One disk was fixed and one was moveable. See here.

In cryptography, a Caesar cipher, also known as Caesar's cipher, the shift cipher, Caesar's code, or Caesar shift, is one of the simplest and most widely known encryption techniques. It is a type of substitution cipher in which each letter in the plaintext is replaced by a letter some fixed number of positions down the alphabet. For example, with a left shift of 3, D would be replaced by A, E would become B, and so on. The method is named after Julius Caesar, who used it in his private correspondence.
The Zodiac Killer may have employed a variation of this technique by using a right shift of 8 in his 13-Symbol cipher, encircling the number 8 to denote the use of a Caeser Cipher Wheel. By placing a wheel or circle around the number 8 in the 13-Symbol cipher, the Zodiac Killer may have been telling us to rotate through 8 positions of the cipher to reveal his name. Of course, the Zodiac Killer was never going to give us his name, but he could have been playing one big joke.
The Zodiac Killer may have employed a variation of this technique by using a right shift of 8 in his 13-Symbol cipher, encircling the number 8 to denote the use of a Caeser Cipher Wheel. By placing a wheel or circle around the number 8 in the 13-Symbol cipher, the Zodiac Killer may have been telling us to rotate through 8 positions of the cipher to reveal his name. Of course, the Zodiac Killer was never going to give us his name, but he could have been playing one big joke.

It can be seen in this image that if you rotate the outer circle clockwise by eight positions from the first number 8, it lands over the M of the inner circle. The second number 8 rotated by eight positions lands over the letter E. The third number 8 rotated by eight positions lands over the crosshairs of Zodiac, giving us the following message shown below. But how can we possibly prove this was the intended message, in contrast to a name or message that used all thirteen characters.
After nearly four years of this code being unsolved, the Zodiac Killer may have thought it time to give us the answer by mailing us the Exorcist letter on January 29th 1974, offering us the words "Signed, yours truley" followed by a verse from The Mikado's Tit-Willow. When you examine the complete verse of Tit-Willow, you will notice eight consecutive letters of the alphabet spelling "My name is".
The only other place this is found is in the introduction to the 13-Symbol cipher on April 20th 1970. In other words, we can only connect the phrase "My name is" to these two communications.
After nearly four years of this code being unsolved, the Zodiac Killer may have thought it time to give us the answer by mailing us the Exorcist letter on January 29th 1974, offering us the words "Signed, yours truley" followed by a verse from The Mikado's Tit-Willow. When you examine the complete verse of Tit-Willow, you will notice eight consecutive letters of the alphabet spelling "My name is".
The only other place this is found is in the introduction to the 13-Symbol cipher on April 20th 1970. In other words, we can only connect the phrase "My name is" to these two communications.
The Zodiac Killer certainly wasn't going to make it easy for us, but if the observations described above are correct, the Zodiac Killer gave us "My name is", followed by his signature of "Me - 37" in the Exorcist letter, in similar fashion to the April 20th 1970 letter, which may have given us "My name is", followed by "Me (and his crosshairs)". In most of his communications the Zodiac used his crosshairs accompanied by a running victim total, therefore, "Me - 37" and "Me (and his crosshairs)" can be equated to "crosshairs - 37", just like his previous running victim totals. Such as the April 20th 1970 letter below. This solution to the 13-Symbol cipher can at least be replicated in a later communication, with the common signature of "Me".
The phrase "My name is Me" is a statement of identity, often used playfully or to emphasize a unique connection to oneself. In the July 31st 1969 letter to the San Francisco Chronicle the Zodiac Killer wrote "In this cipher is my idenity". On August 4th 1969 he wrote "By the way, are the police having a good time with the code? If not, tell them to cheer up; when they do crack it, they will have me".
The phrase "My name is Me" is a statement of identity, often used playfully or to emphasize a unique connection to oneself. In the July 31st 1969 letter to the San Francisco Chronicle the Zodiac Killer wrote "In this cipher is my idenity". On August 4th 1969 he wrote "By the way, are the police having a good time with the code? If not, tell them to cheer up; when they do crack it, they will have me".