(1) I'm interested in buying almost any M1 Carbine, or if you change out parts I'm interested in the parts you take off. They have no originality value to Plainfield and maybe never will. Early parts are GI. Later parts are PMC marked and are usually cast instead of milled. PMC parts are only correct for Plainfield. GI guys don't want them.
(2)This guy, Jim Mock, is collecting info on all commercial Carbines and can tell you when yours was built. He'd like for you to send him a quick email with some machining questions on yours so he can database the progression. You don't have to give your complete serial number.
http://www.m1carbinesinc.com/carbine_plainfield.html
I can tell you what I know about them. The Plainfield was my first Carbine. It's the better, but honestly only the early one is that interesting. By the early to mid 1970s they were the same as an Iver Johnson and were picked up by IJ. All the parts were aftermarket except the barrel, mag catch and maybe flip safety in the 1970s.
However, they started in the 1960s as a new made receiver with '03 barrel with soldered on gas cyl.. The rest of the parts were all USGI surplus except the stock. The Plainfield stock has a black plastic butt plate with 2 screws vs one on GI. Even though they're all USGI moving parts with the PMC receiver you can definitely feel the difference when you cycle the bolt by hand. They shoot just fine if not worn out. Plainfields showed up in police departments and even in returns from foreign nations in cases of mostly USGI carbines. I've heard that they supplied new M1s and M2s to the CIA, but for sure went to one or more South American nations.
IMO, the Alpine is every bit as good as the early Plainfield, but it didn't hang around as long to get the non USGI parts put in it. Plainfield had a little more government history.