The Monticello card was preceded by the 148 character cipher and letter (likely mailed in May 1971) beginning with the wording "I'll do it to because I done it 21 times" and effectively pre-empting the victim total on the Monticello card. It was almost certainly mailed shortly after the beginning of May. Despite the fact the murders of Debra Gaye Furlong, Kathie Ann Snoozy and Kathy Bilek had already been attributed to Karl Francis Werner, it seemed that the Zodiac Killer was determined to suggest otherwise. Or, at the very least, compound the misery of the families and friends of the victims. The Monticello card featured in the San Francisco Police Department DNA testing of possible Zodiac correspondence, so was obviously held in high regard (yet may have contained sensitive material, or featured an upsetting image deemed not suitable for release). If the DMV letter had followed the Monticello card it too may have been considered unpalatable regarding the San Jose and Saratoga murders.
Assuming these communications were mailed by the Bay Area murderer it would be fairly obvious that the Zodiac Killer was following the San Jose murders closely from August 3rd 1969, through his Dripping Pen card of November 8th, to the eventual murder of Kathy Bilek, and up to his correspondences in the middle of 1971. Therefore it would be no great surprise if he continued to monitor any newspaper or magazine articles regarding the murders of the three young females.
One particular section about Kathy Bilek caught my eye - that may have been the reason why the Zodiac Killer wrote a letter to the chairman of the DMV in Sacramento. It stated "At about the same moment, Park Ranger Ken Williamson was informing a member of the investigative team that he, too, had noticed a tall, bespectacled young man who had been a frequent visitor at Villa Montalvo. He was a loner and had impressed the Villa's ranger staff with his almost furtive conduct, said Williamson. The ranger had jotted down the license number of the strange youth's auto. The license number was forwarded to the California Department of Motor Vehicles headquarters in Sacramento, the state's capital 150 miles to the north of San Jose. The information teletyped back from the registration bureau was that the auto was the property of a Karl Francis Werner, living at an address on Shawnee Lane in San Jose".
The Zodiac Killer, having previously mailed two communications (Monticello card and 148 character cipher) referencing the murder of Kathy Bilek and the victim total of 21, it wouldn't be at all surprising if he fired off a third communication, this time to the Department of Motor Vehicles in Sacramento after reading these two paragraphs. As this magazine article was published in October of 1971, the DMV letter may have been postmarked with the latter months of that year - possibly October, November or December. The fascination of the Zodiac Killer in claiming, or interjecting himself into the middle of these murders may have been the driver behind his contact with the chairman of the DMV. The content of this communication is unknown, but I guess he is continuing the menacing theme of his previous two correspondences, suggesting the identification of the vehicle belonging to Karl Francis Werner is somehow another red herring and threatening more murder and mayhem.