California Dept of Corrections and the California Youth Authority (think Juvenile detention) issued 3" Model 64's for a lot of years. When they "upgraded", they released all their old guns into the market. My guess is it's one of those.
I've also got a 3" model 64 marked CYA. It looks almost new and shoots like a dream. I picked mine up at a local gun show. One of my best friends also has one that he bought from the same guy you bought yours from. To me, nothing balances as well as a 3" round butt K frame. I hope you enjoy yours as much as my friend and I do ours.
Mine doesn't look like new but the configuration of this revolver is real nice. This is going to be a good restoration project. I am going to try to make it look like new and shoot it a lot.
I turned mine into a truck gun. Cleaned it up a bit and mounted a holster inside the truck console. With Buffalo Bores FBI load it's a handful but makes a very nice companion.