I don't think it is a standard Model 39 (or pre 39) magazine, e.g. for 9mm. It might be a 7.65 Luger magazine. Hopefully, cougar15 will come along and tell us.
Some time ago I bought a batch of 52 mags from here . To my surprise 2 of them weren't 52 mags , come to find out they were 39 mags . Here's some pics , don't know if they'll help .
I cant tell from the fuzzy pic of the base plate if it is S&W stamped or caliber marked.
It could be a exp-tool room model. I will say not a factory production mag.
The 7.62 mag is just the std. 9MM mag.
The magazine doesn't look like any pre or M39 mag I have ever seen and it's not for a M52. The baseplate and sides are completely different when compared to those that are shown in post #4 and look at the differences in the way the baseplates attach. The only thing on the mag that even looks like the M39 is the welded X follower. Do you have a M39 mag to compare it to or can you provide some measurements? Have you tried to load any 9mm ammo in it?
Definitely not a M-39 magazine, and not even a S&W magazine with that style of floor plate and the magazine sidewalls bent in to retain the floor plate. The only thing even similar to a M-39 magazine is the X-follower.