I really tried. Including google image searches. It looks at least 25 years old to me maybe even an aftermarket magazine of some type.
Came up with nothing. I find the checkering on the bottom odd. Don't get too hung up on it in your search, it too could be a long forgotten aftermarket replacement part. The best tell might be the thumb recess in pic #2 on the magazine base plate, if you can find it again that should give it away.