The Red Phantom letter ends with "Meanwhile, cancel the Count Marco column. Since the Count can write anonymously, so can I". The article on the left ends with "You don't need psychiatry. All you need is the Count Marco column". Hence the reference to "column" in the letter.
Marc H. Spinelli wrote this column under an anonymous pseudonym - and why the author of the July 8th 1974 communication used the word "Phantom". A phantom can be described as a ghost, a figment of the imagination, not real or illusory. The author was simply choosing a pseudonym lacking visibility and substance. Although, The Phantom was an American adventure comic strip published by Lee Falk in February 1936 that has run in many newspapers to this day, syndicated by King Features of Hearst Holdings, Inc.
The addition of "Red" before "Phantom" explained perfectly by the bracketed "red with rage" placed underneath. The Count Marco column had already detailed in the first paragraph that he "wasn't surprised by the angry reaction" when he wrote about psychiatrists and psychologists, so it's probably no surprise that the author of the San Rafael letter used the word "Red" before "Phantom" to express their anger or rage. No external "Red Phantom" was required as inspiration for this correspondence, because every element of the correspondence was derived from within the July 3rd 1974 Count Marco column. The Red Phantom is just a construct of the author's mind, that has ultimately become a figment of our imagination.