What is even more amazing, is that the Zodiac Killer began the SLA letter with "Dear" and ended the communication with "a friend". This letter was not published in the newspapers. Despite this, the Symbionese Liberation Army typed a communication to the Hearst family just seven days later, on February 10th 1974. It too began with "Dear" and signed off with "A friend". Not only was the Zodiac extremely fortunate to time his SLA letter the day before Patty Hearst's kidnapping, but seven days in advance, he predicted the opening and ending words in the February 10th 1974 Symbionese Liberation Army letter. The sarcasm dripping from my internet keyboard should be obvious, that I don't believe the SLA letter was authored by the Zodiac Killer - and neither was the Red Phantom letter.
Because the SLA letter and envelope were not released to the public at the time, the author of the Red Phantom envelope had an uncanny knack of imitating the handwriting and format of the SLA letter. These are with little doubt the same author. If the Zodiac Killer was not responsible for the SLA letter, then he wasn't responsible for the Red Phantom letter either.