To his credit, notable Zodiac researcher Cragle unearthed an article in The Napa Register on December 29th 1969 that dispelled this notion (see below). But when I proposed the idea - asking people to check if a newspaper image of the Melvin Belli letter existed - it took two years to achieve. This is with the combined power of the Zodiac internet community, and a comprehensive newspaper database at our fingertips.
The April 24th 1978 letter, stating "I am back with you. Tell herb caen I am here" was postmarked only 42 days after a voice message was left on the telephone answering machine of an individual living in the Mission District, using similar wording to that used in the letter, and claiming to be the "Zodiac". The telephone caller stated "This is the Zodiac. Tell the press I am back in San Francisco". Read more. Was this the Zodiac Killer twice, or the hoaxer twice?
Thanks to Cragle, another unusual mention in the above newspaper article may be relevant to 1978. It may indicate that the person who crafted the April 24th 1978 and May 2nd 1978 letters were one and the same - who had kept or read The Napa Register publication. The article above finished with the following text.