Thanks for the replies all. I bought this gun about 10 years ago at an Oklahoma Gun Show. It came with a replica folder stock with USGI steel on it. So basically you are saying that the real value is in completely original carbines or nearly original is where the value is at? If that is true I will end my quest to out fit this one with NPM stamped parts. I am going to need another barrel in the future. The S/N on my receiver is 4106xxx and the barrel is an Underwood marked 10/43. I have a gas leak at the piston nut due to loose threads. The piston and nut are new. I took it to a welder to weld up a small crack near the clean out hole. He advised me to clean out the piston chamber real well which I have done and then while slowly screwing in the piston nut apply red thread locker in the clean out hole to fill up the threads, then let it sit barrel tip up for 24 hours. I haven't fired it yet but plan to this week weather permitting. The recoil spring is new. My Gunsmith fired it several times and it fails to return the slide all of the way back to catch the next round in the magazine. He attributes this to gas leaking past the threads. Thanks much for you help. Magnafique
This is a tough question, about value. There are those who will attribute greater value to a rifle that has all NPM parts. Those are the people who think NPM carbines came with all NPM parts; they didn't. Then there are those who are looking for an "original condition" Carbine, and those people all have their idea what "original" is.
M1 Carbines hold value pretty well regardless of configuration. Will your NPM have more value with NPM parts, probably.
But understand that M1 Carbines were made with parts from about 40 different subcontractors, and ALL of the manufacturers used parts from those subcontractors. So that begs the question, what is "original"? To be true, most any configuration M1 Carbine from first models to first arsenal upgrade could be "original". A true "original" M1 Carbine will have all of the original Parkerized finish, and can have most any combination of parts. Any rifles made before mid 1943 should have the flip up sights, push button safety, first or second model magazine catch, flat bolt, non-bayo lug barrel band, first style recoil shield, flip up rear sight, flat bottom stock, etc. From mid-late 1943 it could include any of the parts found on the first arsenal upgrade, as well as original parts. (second style magazine catch, lever style safety, late style firing pin, second style recoil shield, second style hammer, second style slide, machined fully adjustable rear sight, bayo lug barrel band, etc).
To know what is what, pick up a copy of the M1 Carbine Technical Manual (TM9-1276) : http://www.90thidpg.us/Reference/Manuals/TM9-1276_1947.pdf
The manual has very good photos and descriptions of each part, early/late, etc. and explains the differences (things like early hammer springs had 22 coils, later ones had 26 coils, etc).
So regarding value, there are the "collectors" who will be in the market for an "all original" NPM with all the NPM parts. They either don't understand that there never really was such a thing, or they're just willing to just agree amongst themselves that an all NPM parts NPM Carbine is "correct". M1 Carbine collectors are a strange bunch.