Is the 2nd digit truly indicative of the date of issue? In my case the car has an 'earlier' tag than yours but registered 2 months later.
My early 1966 GT350 retains its original CA black plates (RNTxxx). It was sold new out of Hayward Motors in LA on January 21, 1966. I'm fortunate to have all the original sales docs as well as virtually every receipt, registration card, bill of sale, repair order, etc. since new.