I have collected M1 Carbines for years. There are a few things I would tell anyone thinking of collecting these. First, the number of "paratrooper" carbines that are out there is pretty small. You should assume that anything called a paratrooper carbine is a fake put together by Bubba in his garage. This is a general rule, but generally true.
Second, some collectors, especially guy's trying to sell carbines at gun shows, will try and claim a carbine is "correct", meaning it has all parts by one particular manufacturer. This is unimportant to me, and should be unimportant to anyone who knows the history of the carbine. Not all manufacturers made all parts. Of the manufacturers who did, it was not unheard of for them to use small parts made by a different manufacturer. In other words, many carbines left the factory with mismatched parts.
Third, when a carbine was re-arsenaled, no attempt was made to keep all the parts original. Everything got mixed up.
So if you see an all original, correct M1 Carbine at a gun show, the chances are that it was made that way by Bubba in a garage pulling parts of one manufacturer's gun and putting them on another til he got all the parts matching.
Very few carbines that saw military service should have parts by only one manufacturer.