Hello Tom, Not sure if this answers your question or not but I will give it a try. California used to issue new plates every 6 years or so. The "Black plates" were first issued in 1963 and "63" was stamped into the plate. As I recall, the yearly re-registration was required Jan 1 of each year and a new sticker with the appropriate year was issued and placed on right side of the plate. In 1970, the 1st "Blue Plates" were issued and at that time it was decided to not reissue new plates for all cars and start the registration year from the date the car was sold and month sticker was applied to the left side of the plate. For cars that still had the old black plates, new re registration dates were spread out over a 12 month span so even though my car was first registered in Sept 1968, my renewal now comes up in March every year and have to apply new yearly stickers every year since. Every six years or so, I carefully peel away the layers of old stickers. They would be an inch thick by now if I didn't:-)
Hope this makes sense.
John
BTW, My family purchased a new car in January 1971 in LA area and the last three letters, on the blue plate, were CJT