Whoa! Can't let that go by r_taylor, you are SO wrong. Most of the WW II builders of M1 carbines made, or had made for them parts that were marked with their unique code for their own carbines,. Example "W", for Winchester, "I" for Inland, "B" for IBM and so on. Most carbines in WW II came out of the respective factories with matching parts, thus coded, with a few exceptions where parts were shared among companies. War Baby series by Larry Ruth, or Riesch book on wartime M1 carbine production are good start points for research.
Some of the contractors, Quality Hdw, Saginaw S'G', and National Postal Meter didn't make any barrels, and used free issue from Underwood, and others. Marlin & Buffalo only made barrels, and are found on NPM, STD PRO, and QH, I-P, S'G', NPM, and STD PRO respectively, although Underwood is the most common for these makers. Winchester, IBM and Rock-ola also shared barrels with others too. "P" proof marks for any of these barrels are unique to the main contractor, ie a "P" stamp on an Underwood barrel on an NPM is different to a "P" stamp on an Underwood barrel on a S'G carbine.
Retro-built carbines are easy to spot, staking marks on the dovetail, wear marks on the barrel from Type III barrel bands, and more.
Mixmasters came about, post war, due to upgrades and rebuilds to upgraded parts, namely adjustable rear sights, bayonet Type III barrel bands, and flip safeties. Some with T-17 kits to make them select fire, like the M2 carbine. Stocks were stamped with letters to indicate which US arsenal did the upgrade, except FN in Belgium which used no stamping at all.
For a good website on German/Austrian use of M1 carbines by my colleague is at
http://www.bavarianm1carbines.com/
Germans stamped the last 4 digits of the original S/N on the stock, bolt, slide, and a few other parts.
Bavarian carbines with notched rear sight are around the same retail value as as any restricted M1 carbine, depending on condition, $600-$800 CAD.