On December 29, 1969, the office of the FBI in San Francisco sent a list of twenty individuals provided by the police to the Identification Division (here’s the airtel) for a check in the Zodiac matter. In their response to this request dated 12 January 1970 (reference), the experts of the Identification Division pointed out that the suspects in the 29 December list had «same or similar birth dates» (here).
Probably after the response of the FBI laboratory, noting the experts’ observation, the agents in San Francisco asked the SFPD an explanation to the suspects’ birth dates. Then, on 14 January, they reported to the Director and Identification Division that a self proclaimed Zodiac had called attorney’s Melvin Belli’s home saying it was his birthday. They did not specify the date of the phone call.
“TODAY’S MY BIRTHDAY”
On 20 Decemeber 1969, the real Zodiac mailed a letter to Belli at his home in San Francisco. The murderer wrote: «I cannot remain in control for much longer» (letter). On the same day, a year before, he had killed a couple of teenagers on Lake Herman Road.
20 December is also Joe Bevilacqua’s birthday and the last day of staying in the newsstands of the issue of the magazine used by the Monster of Florence to compose his letter to Della Monica, in 1985.
In the same period of time in 1969, a self-proclaimed Zodiac phoned attorney’s house. Not finding Belli, because in Europe, the man told his housekeeper, Erna, something similar to what Zodiac had written: «I can’t wait.» Then he added: «Today’s my birthday».
AT THE END OF DECEMBER
Both Zodiac and the anonymous caller had tried to contact Belli at the same address, in the same period, asking for haste towards them using an analogous expressions.
According to the San Francisco Chronicle, the Belli letter was opened only after Christmas and the news did not become public knowledge before December 27, which almost completely excludes that the call had been made by someone who, other than Zodiac, was aware of his message to Belli. This supports the hypothesis that the call was made by the real serial killer.
According to this source, the date of birth of the mythomaniac who had called Belli on television in October 1969 and would be traced by the FBI after another phone call to the KGO-TV (FBI communication) did not fall within this period of time.
Even if the public does not know for sure if the man who called Belli was Zodiac and on what day the phone call occurred, December 20 seems to be the most probable date for the “birthday call”. To verify this deduction, the police could check in the list of 29 decemeber the date of births redacted from the public copies for privacy reasons.