Unless there has been crossed wires, and the communication by Paul Avery requesting "ZODIAC" license plates from the Department of Motor Vehicles is the mysterious DMV letter, then a yet to be unearthed letter mailed by the Zodiac Killer in late 1970 is a distinct possibility.
A big thanks to Zodiac researcher Jibberjabber, who suggested this as a realistic inspiration for the elusive DMV letter. Also, I would like to thank Jarett Kobek for his assistance in allowing me to use a screenshot from his book, Motor Spirit: The Long Hunt for the Zodiac, and for sending me the newspaper cutting from Herb Caen's column on November 25th 1970. Jarett Kobek's book, How to Find Zodiac, can be found by clicking the link provided. The personalized license plate article provided is courtesy of Cragle.