I will have to check my U.S. .45 ACP magazine's and see if what I have for the letter stamps.....
WW2 production magazines came from Colt, Little, Risdon, General Shaver, and American Pin Company. Colt-produced magazines were generally unmarked; Colt also delivered pistols with Little and Risdon magazines made on contract, marked C-L or C-R. Little produced magazines for Remington-Rand, Ithaca, and Union Switch & Signal. I think just about all the General Shaver mags were "spare parts", shipped out as replacements to be issued as needed. American Pin produced relatively few during WW2, but were one of the more common makers for WW1 pistols.
After WW2, as replacement magazines became needed, several companies produced them. Those are generally marked on the floorplate with the part/stock number and a manufacturer's code (there are some on-line resources for deciphering those markings to identify maker and contract time period). Generally not considered very collectible, for now anyway.
As recently as 2005 I was still finding new, unissued WW2 magazines still in the original VPI paper wrappers. Picked up an unopened box of 24 General Shaver mags at a gun show, made several collectors very happy while also lining my pockets nicely.
There are literally millions of the older magazines still out there, frequently in the hands of folks who have no idea what they are holding. I still keep my eyes open for deals, both to build my own collection and to provide extra funds for other "needs".
An interesting sideline field of collector interest.