This is my first post and will post pics soon of my new 66 c-10 fleetside (my uncles.... pretty good story)

Just got the truck home and put a registration sticker on it, but when peeled off all the old stickers I noticed the plate said 63 engraved(on the year section) Plate is a black CA plate....What does the 63 refer to. all other paperwork has truck as a 66.

Any info would be appreciated....Plus I'll have a ton more questions....Looking forward to the restoration