I do not believe these are STONER magazines. Stoner mags are a totally separate type of magazine.
I was first exposed to these type of magazines November 1988. They were try/use experimental (marked "X" mags) issued during a REFORGER exercise. They did not work well because the tiny curve created a spring binding problem when loaded to maximum. Colt 20 round straight mags did not have this problem.
The next time I saw them was at the PMC range pre 9-11 shotshow demo. I think Bushy had them there, and again, the spring slipping/binding issue reared it's ugly head. I have never seen them since.
I hope this helps.
V
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