Due to the fact that 15 rd surplus mags have been comming out of our ears until quite recently , so for about 70 years now , it has been knocked around by many experts that the US produced a box of mags ( 100 count) for every carbine. I have one unsealed complete case ( it was in a sealed bag. I then removed bag layers from the '80s , the 60s , and the 50s , till I came to a "V for victory " box of Union Hardwares. As I stared at it , the glue on the lid just poped and it raised up like in a Sci-fi movie. Ergo , it un-sealed itself. ) and about 2-3 dozen "user" 15 rd and a dozen or so 30 rd. , so that puts me at around 10 per carbine too.
Chris