The S.L.A letter was postmarked "U.S. Postal Service, CA 913 PM 3 FEB 1974". This tells you unequivocally that the S.L.A letter was mailed from Los Angeles County, and was likely posted on February 2nd or February 3rd - a day or two before the Patricia Hearst kidnapping by the Symbionese Liberation Army, on February 4th 1974.
I hope that the 'narrative keepers' maintain a February 14th 1974 mailing for the S.L.A letter, because then they have to explain how the Zodiac Killer reappeared after nearly three years of hibernation, to then write a letter about the Symbionese Liberation Army, beginning the correspondence with "Dear" and ending with "a friend", only four days after the actual Symbionese Liberation Army mailed a letter (not publicized) to the Hearst family on February 10th 1974, beginning the correspondence with "Dear" and ending with "A friend". Now that is one hell of a coincidence.
Kevin Robert Brooks created an extremely credible solution to the Exorcist letter symbols in the form of the words "To Kill". In view of the preceding paragraph, his solution is incredibly convincing: "If I do not see this note in your paper, I will do something nasty, which you know I'm capable of doing. To Kill". Whether this is the answer or not, it is certainly a powerful argument, when we consider the S.L.A letter was entirely geared to giving us the word "Kill". This seemed the only purpose behind the S.L.A communication - as though the letter was designed to give us an answer. The fact that the Exorcist letter symbols were withheld from the January 31st 1974 San Francisco Chronicle publication, may have been the inspiration for the February 3rd 1974 correspondence. It was certainly very fortunate that "Kill" could even be created by the symbols at all, and subsequently highlighted in quotation marks, a matter of days later.