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XCR pistol mags, they're the only ones I've seen.
Almost all of mine needed tweeking at some point in their life, re-shaping the feed lips. They also needed trimming of the follower legs, or they'd be 9.5 rounds.
I've got 21 of them, iirc, haven't had to throw any away yet.
I just bought a 5/20, don't recall brand. Looks good, runs like poo. It's a 20 round 5.56 mag with x39 follower, crimped to 5 of x39. I use it for pictures .
Edit:
The mag looks the same as a 20 round 5.56.
It is purpose built for 7.62x39, with different feed lips.