The plates must be from the year of mfg. So if you have a 64 the plate letters needs to be issued that year. My Phase One was mfg-ed in 63. They wouldn't go for my arguing the plate was stamped 63. It is considered a 64 model car. It can also have the white year sticker that says 64 instead. It you bring in a plate that has a 63 stamped on it alone, that will not do. The problem is that you do not know what is at the bottom of the pile of stickers that accumulated. Taking them off will ruin the bottom sticker. There is a number to year chart floating around. That is the best way. Your plates need to be serviceable and you must have both plates. They sell for about 350 at flee markets for nice ones. Sunday, I was at a giant auto antique car parts flee market in Sacramento (Arco Arena or what ever they call it now) where there were 1100 vendors. Lots of old plates but the cheap ones were in poor condition. There are people who for a price ($$$)can refurbish a pair. Try E-bay but know the letter combo before you bid.Good luck. It took me 2 years and lots of letter writing. The state legislators were on board but the head of the DMV was apposed. He wanted reflective plates. I talked with many clubs and the Hot Rod club helped move things I wrote Jay Leno but that did nothing. The Hot Rod guys got Jay's help and thing happened.
If you send me a PM, I may have something that will help you out.