Zodiackillersite.com

Not very much discussion about the fairfield cipher, so far.
http://www.zodiacciphers.com/the-fairfield-letter.html

The cipher can be read eather as a five-letter word, a six-letter word or a 4-letter combination with two additional letters. Focussing on the first two, the following solutions are possible:

CLEVE STEVE REESE REECE RONAN
STEVEN EVERET GERARD GRAHAM PHILIP

Other words than names are possible, however it doesn't make very much sense to sign such a letter with e.g. the word 'basis'.

Also city names could be a nice 'clue':

EXETER (next to Visalia)
COTATI (next to Santa Rosa)

Not sure if that will lead to anything substantial.

QT

This letter and the cipher are discussed in the section for Unconfirmed Letters, in the thread called "This State Is In Trouble".

The writer of this unconfirmed (probably fake IMO) letter also probably wrote another letter with a Fairfield postmark. This was the letter of 12/7/69, that says "I need help", and states he will kill again "+ he will be a cop". That letter also has a short cipher.

A code this short, six symbols, that used a unique key would be unsolvable. So I applied the 408 Harden Key and came up with:

S I N H N D

Or:

S/Y I N H N D