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' code 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.
Could the killer's initials be FK or KF, on account of the upside-down text on the '13 Hole' postcard, or could the killer simply want us to believe this is the case.
The FK in close proximity occurs seven times on the '340' cipher, but much more significantly, the eye is drawn to the 'mistake' on line six, where the author of the cipher elevates the letter K in reverse, next to the letter F. The author of the '13 Hole' postcard effectively rotates both letters through 180 degrees, again reversing the letter K.
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. http://www.zodiacciphers.com/zodiac-news/340-cipher-and-exorcist-link
If the Zodiac Killer lived within a 10 minute walking radius of the Springs and Tuolumne payphone, then by examining land registry records in 1969 for males with FK or KF initials and cross checking their vital statistics, including photographs, would go a long way to deciding whether this is just another pattern, in a rich tapestry of Zodiac imaginings.