This is interesting. I don’t think I saw this mentioned in the thread but The Red Phantom letter was mailed from Marin County.

Just my Opinion

THE LETTERS

From what has been deduced by a number of different posters we know the following about the letters :-

Sent from Berkeley, Ca – March 1968 – to Syracuse, NY
FBI Looking into letters in April 1970
Selective Service number – Ascertained that writer was born in 1947
Multiple References to Tiburon
The contents of the letters are the either intentional or unintentional ramblings of a heavy drug user (LSD) trying to justify their reasons for not being called up for service. I do not want to go into the context of the letters as that will come up later.

THE WRITER

I think am 75% certain that these were written by Herbert Mullin.
He was born in 1947 as per the selective service number.
In January 1968 he registered as a conscientious objector, he granted this in October 1968 after his father wrote a letter to the draft saying how “peaceful minded” Herb was currently. These letters are from the time period between these two events.
The Month that they were written he broke the engagement with his longtime girlfriend Loretta . He started to become violent and said he was bisexual.
In late 1967 Herbert consumes himself with the study of Hinduism, reincarnation and yoga.
28th March 1968 he was diagnosed as a schizophrenic, aggravated by drug use.
His writing in unmistakably similar to the letter writers as can be seen below:-

WHY 1970 ?

As we Herbert Mullin did not start his killings until 1972 so why would the FBI be looking at these letters two years before that ?
The devil himself was also in Tiburon at this same time, Charles Manson, his former parole officer was a Roger Smith (Manson was Smith’s final prole client before quiting). The two had actually grown close, in fact in June 1968 Smith even gave glowing appraisals and petitioned for leniency when five girls from the family were arrested for drugging a group of underaged boy’s. They even fostered Mary Brunner’s / Manson’s son when she was imprisoned at this time.
Interestingly Smith wasn’t a career PO; he was a doctoral candidate at the Berkeley School of Criminology, focusing on the link between drug use and gang violence. Roger Smith had him reporting for his parole appointments, his weekly appointments at the Haight Ashbury Free Medical Clinic in the summer of ’67, where he was also preparing a study on amphetamines and communes in the youth of the Haight Ashbury during the Summer of Love.
Manson even stayed at their Tiburon home for a time when he was homeless.

So I think it stands to reason that why the FBI looked into this letter was due to the numerous Manson connections to it.