The October 5th 1970 13-Hole Postcard has links to the 13-Symbol cipher for obvious reasons, but the author of the postcard does paste some unusual text, notably "Fk, I'm crackproof. What is the price tag now?" Is there a correlation to the 13-Symbol cipher? Immediately after "Fk, I'm crackproof", the author asks about the price tag on his head. Immediately after the 13-Symbol cipher on the April 20th 1970 correspondence, the author writes "I am mildly cerous as to how much money you have on my head now". So what is the likelihood "Fk, I'm crackproof" precedes this question also. It would make sense when the April 20th 1970 communication is read as a whole: "This is the Zodiac speaking. By the way have you cracked the last cipher I sent you. My name is FK, I'm crackproof". The phrase itself is 14 characters, so would be infinitely circular in configuration, beginning and ending with the letter F, like so.
The Exorcist Letter mailed on January 29th 1974 looks somewhat similar to the sixth line of the 340 cipher, in that the strange symbolism at the foot of the letter appears to marry up. It contains what looks like a letter F, and to its elevated left is an upside-down K. But there is much more correlation, in that five of the characters on the Exorcist letter bear a striking resemblance to five of the characters by the FK on the 340 Cipher. 340-cipher-and-exorcist-link