In the FBI file below, it states "Enclosed for the Bureau is one photocopy and one xerox copy of letter and envelope received 2/26/74 from Inspector (redacted) Homicide Detail, San Francisco Police Department. Enclosed for Sacramento is one xerox copy of same letter. For the information of the Bureau, on 2/20/74, Inspector (redacted) advised that on 2/14/74 the enclosed letter was received by the San Francisco Chronicle and (redacted), reporter for the San Francisco Chronicle brought the letter to (redacted) as a possible Zodiac letter. (Redacted) thereafter turned the letter over to (redacted) questioned document examiner, United States Postal Office, San Francisco. Mr (redacted) has advised (redacted) that he feels the letter is a Zodiac letter".
What the above document doesn't tell you - is when the S.L.A letter was mailed or postmarked.
The above document read "that on 2/14/74 the enclosed letter was received by the San Francisco Chronicle." Received is not mailed. If we travel a little further in the FBI files we can find more information on the S.L.A letter and envelope, listed under Qc64 and Qc65, as circled in red on the above document.
The FBI files state on more than one occasion:
Qc64 Photocopy of envelope postmarked "U.S. Postal Service, CA 913 PM 3 FEB 1974", bearing the hand printed address "Editor San Francisco Chronicle San Francisco, California".
Qc65 Accompanying photocopy of sheet of paper bearing the hand printed message beginning "Dear Mr. Editor, Did you know that the....."
According to the FBI files above, the S.L.A letter was mailed on or before February 3rd 1974 (postmarked 3 FEB 1974).
Therefore, it was likely mailed one day before the Symbionese Liberation Army kidnapping of Patty Hearst. This means one of four things:
 The Zodiac Killer was a member of the S.LA. and was involved in the kidnapping plot.
 The Zodiac Killer didn't belong to the S.L.A, but somehow knew of their plans to kidnap Patty Hearst.
 The Zodiac Killer just got extremely lucky, writing "Dear Mr Editor, Did you know that the initials SLAY spell "sla", an old Norse word meaning "kill". a friend", then mailing the letter on February 3rd 1974, just one day before Patty Hearst was kidnapped by the Symbionese Liberation Army. In other words, it was sheer coincidence. Or,
 The letter was written by the Symbionese Liberation Army and had nothing to do with Zodiac.
The emphasis on the word "Kill" at the foot of both letters, within days of one another, leading me to believe they are connected. If the S.LA. letter was penned by the militant Symbionese Liberation Army on or before February 3rd 1974, then a good case can be argued, that they were responsible for the Exorcist letter too.
S.L.A LETTER NOT ZODIAC - AND HERE IS THE FBI FILE TO PROVE IT