Had the Zodiac Killer set aside three portions of shirt to send, it is highly unlikely he would want to be sitting on incriminating evidence for too long - and after the murder of Paul Stine in Presidio Heights on October 11th 1969 he would mail his three portions of shirt with his next three correspondences. His first being the October 13th letter, just two days after the murder, his next letters were postmarked within one day of each other, so he sent the second portion with these, with the final piece being mailed with the Melvin Belli Letter on December 20th 1969. What he didn't do however, was mail a piece of shirt with the Fairfield Letter on December 16th, four days before the Melvin Belli Letter.
However, newspaper accounts were starved of the full details, because the 'by knife' attribution on the car door of Bryan Hartnell's car was withheld from the public in an attempt to flush out the real killer from any subsequent hoaxers - and the Fairfield Letter may be one example. Anybody who was aware of the Lake Berryessa attack could have concocted the phrase 'The Bleeding Knife of Zodiac' - and more importantly its failure to highlight 'by knife' was an opportunity missed from a killer now determined to link himself to the authenticity of his correspondence by enclosing the swatches of Paul Stine's shirt.
The Zodiac Killer had his ego dented when his first cipher was cracked in a matter of days by school teacher Donald Gene Harden and his wife, Bettye June Harden, two amateur sleuths. His response was to send us the still unsolved 340 Cipher, crafted either with no solution, or a solution that went far beyond simple homophonic substitution. The Fairfield Letter, however, seemed a retrograde step into mediocrity, when the supposed Zodiac Killer mailed just nine symbols, showing little effort in the apparent guise of a small code and beginning his letter with "I just want to tell you this state is in trouble, I will go for the government life". This appeared more like a politically motivated correspondence, with bold claims of another potential 38 victims. The 'knife' wording does look similar to the writing on the card door at Lake Berryessa, but the template for these letters was freely available throughout his correspondence up to this point. The one thing that is apparent, is his failure to corroborate this letter with a swatch of Paul Stine's shirt, that has led some people to believe the Fairfield Letter was written by the hand of a hoaxer and the real Zodiac Killer was to contact authorities just four days later.
It is widely acknowledged that two pieces of Paul Stine's shirt were sent - firstly to the San Francisco Chronicle on October 13th 1969, just two days after his murder and secondly on December 20th 1969 to the home residence of prominent attorney Melvin Belli - but a third piece was mailed along with the Bus Bomb Letter on November 9th 1969, outlined in two articles written by the San Francisco Chronicle and mentioned in a Youtube documentary. Anyone can believe in mistakes, but the wording in the Chronicle is specific. Additionally, the mailing of three portions of Paul Stine's shirt makes perfect sense, as shown in photographs of the shirt on the Zodiac Killer Facts website, marked A, B and C.
Here is an excerpt from the FBI files (Zodiac 5 page 46/249), clearly showing three shirt pieces were mailed.