The SLA letter was postmarked "U.S. Postal Service, CA 913 PM 3 FEB 1974". This tells you unequivocally that the SLA 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 SLA 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.