On November 21st 1969, somebody claiming to be the Zodiac Killer threatened a widow in San Jose, resulting in 24-hour protection given to the woman in question. The FBI file read "For the information of the Bureau and Sacramento, after a cursory analysis, officers of the San Jose, California PD, believe widow in message may be Mrs (redacted), San Jose, California. San Jose PD instituting 24 hour surveillance on Mrs (redacted)". It is fairly evident by the wording in the FBI file that the address of the woman wasn't given by the sender, hence the cautionary wording in the passage of "believed to be the widow". If this threatening letter was mailed by the Zodiac Killer, it is extremely likely he plucked this woman's name from the newspapers. Therefore, we should be looking at a recent widow, either connected to a Zodiac investigation, or independent of a Zodiac investigation, chosen because she featured in recent newspaper articles (possibly in November), that was relevant to Zodiac. Also, by choosing somebody to threaten in San Jose only thirteen days after his claim of involvement in the August 3rd 1969 San Jose murders of Kathie Snoozy & Debra Furlong, the Zodiac Killer could maintain the threat in San Jose and so bolster his involvement in that region. The following analysis will highlight one possibility for the woman in question, but not outright claim she was the widow featured in the November 21st 1969 letter. This possible connection was unearthed by the fantastic work of Cragle, a contributor to both main Zodiac forums, who trawled the newspaper articles in the run up to the mailing of this threatening letter.

James Albert Pike (born February 14, 1913 – died September 3–7, 1969) was an American Episcopal bishop, accused heretic, iconoclast, prolific writer, and one of the first mainline, charismatic religious figures to appear regularly on television. In 1966, after they had shared a sabbatical study at Cambridge University, Pike's son, Jim Jr., fatally shot himself in a New York City hotel room. Shortly after his son's death, Pike reported experiencing poltergeist phenomena—books vanishing and reappearing, safety pins open and indicating the approximate hour of his son's death, along with half the clothes in a closet disarranged and heaped up. Pike led a public pursuit of various spiritualist and clairvoyant methods of contacting his deceased son to reconcile. In September 1967, Pike participated in a televised séance with his dead son through the medium Arthur Ford, an ordained minister in the Disciples of Christ church. Pike detailed these experiences in his book The Other Side (released on January 1st 1968). In 1968, in defiance of C. Kilmer Myers, the Bishop who'd succeeded him, he married Diane Kennedy, a Methodist student twenty-five years his junior, with whom he had collaborated on The Other Side. Wikipedia
The Other Side, detailed James Pike's attempts to contact his dead son in the afterlife, described "as a moving narrative of a father's efforts to save his son from enslavement to psychedelic drugs, a tragic story of a young and gifted man's premature death, a startling story of poltergeist occurrences that led the father and other witnesses to believe that from beyond the grave his son was trying to get in touch, and a detailed account of how this communication proceeded during a time when the father was under accusation for heresy for believing too little".
She became Diane Kennedy Pike, an American writer and educator who lived in San Jose at the time this threatening letter was mailed. She was a school teacher at Willow Glen High School in San Jose between 1962 and 1964. It has been argued on this website that the Zodiac Killer made persistent threats on school teacher Daniel Williams, by way of threatening phone calls to his residence in Martinez and subtle references in the Zodiac Killer's two previous November communications, starting on October 23rd 1969 and ending on November 9th 1969.
Having recently become a widow after the tragic death of her husband in Israel in September 1969, Diane Kennedy Pike was featured in many newspaper articles in the intervening period before the November 21st 1969 "Zodiac" letter arrived. In August 1969, Pike and Diane traveled to Israel, to do research for a proposed book on the historical Jesus. They attempted to travel to Qumran but during their journey through the Judean Desert they got stuck in a deep rut on a dirt road. After leaving the vehicle they went in search of help, with Diane ultimately separating from her older husband (who was in a serious condition). She eventually found help and a rescue party began the search. Unfortunately, it came too late for James Pike, who was found deceased five days later on September 7th 1969. By targeting this widow in San Jose (because of her history and connections), the Zodiac Killer may have thought this would secure him more front page coverage. However, this communication seemingly fell by the wayside - and as of writing - no newspaper coverage of this threat has been uncovered. The Other Side, featuring the afterlife, by the high profile authors of Diane and James Pike, along with his recent death in Israel and the publicity that surrounded it, may have been the required incentive for the Zodiac Killer to threaten this widow in San Jose on November 21st 1969. But until the FBI file is unredacted, we will never know for sure.
The Other Side, detailed James Pike's attempts to contact his dead son in the afterlife, described "as a moving narrative of a father's efforts to save his son from enslavement to psychedelic drugs, a tragic story of a young and gifted man's premature death, a startling story of poltergeist occurrences that led the father and other witnesses to believe that from beyond the grave his son was trying to get in touch, and a detailed account of how this communication proceeded during a time when the father was under accusation for heresy for believing too little".
She became Diane Kennedy Pike, an American writer and educator who lived in San Jose at the time this threatening letter was mailed. She was a school teacher at Willow Glen High School in San Jose between 1962 and 1964. It has been argued on this website that the Zodiac Killer made persistent threats on school teacher Daniel Williams, by way of threatening phone calls to his residence in Martinez and subtle references in the Zodiac Killer's two previous November communications, starting on October 23rd 1969 and ending on November 9th 1969.
Having recently become a widow after the tragic death of her husband in Israel in September 1969, Diane Kennedy Pike was featured in many newspaper articles in the intervening period before the November 21st 1969 "Zodiac" letter arrived. In August 1969, Pike and Diane traveled to Israel, to do research for a proposed book on the historical Jesus. They attempted to travel to Qumran but during their journey through the Judean Desert they got stuck in a deep rut on a dirt road. After leaving the vehicle they went in search of help, with Diane ultimately separating from her older husband (who was in a serious condition). She eventually found help and a rescue party began the search. Unfortunately, it came too late for James Pike, who was found deceased five days later on September 7th 1969. By targeting this widow in San Jose (because of her history and connections), the Zodiac Killer may have thought this would secure him more front page coverage. However, this communication seemingly fell by the wayside - and as of writing - no newspaper coverage of this threat has been uncovered. The Other Side, featuring the afterlife, by the high profile authors of Diane and James Pike, along with his recent death in Israel and the publicity that surrounded it, may have been the required incentive for the Zodiac Killer to threaten this widow in San Jose on November 21st 1969. But until the FBI file is unredacted, we will never know for sure.