Use this site: http://www.eqth.net/mirror/carbinemags/#rock
and compare the markings on the magazine to examples there. If it's marked with a ' mark on either side of the lettering, I can't find any magazines that were made with such marks.
You have to remember, carbine magazines, as well as most any part for an M-1 Carbine, continue to be made in foreign countries. And, though it's sad as hell, there are people who buy these foreign parts, mark them exactly like actual USGI parts, and sell them as genuine USGI parts.