I can’t tell you what gun it fits, but it appears to the the same basic double stack mag body design used by the Sig P226/P228, Beretta 92-series, Walther P88, P99, and numerous other pistol models.
None of these are interchangeable, but the basic body design is the same, and Mec Gar made OEM and aftermarket mags for a lot of them. Perhaps this is some ban-era limited capacity version?
I have a surplus P226 that came with a standard-capacity OEM SIG “dovetail” mag with a similar chunky rubber baseplate to that one, except it was red. It appeared to be an aftermarket addition.
I’m pretty sure this is not a P226 mag, though. On a second look, I believe it is a magazine for some variant of CZ-75 or CZ-75 clone (of which there are many).
For example, take a look at this photo of a modern Jericho 941 magazine and note the similarities of the follower/feed lips/etc.
https://www.gunclips.net/je9419mm16ro.html
Don’t hold me to that, though.
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