If the large rectangular hole in the back of the mag is the mag release catch and you don't have some sort of giantism, the indexing tabs on the side means it's not a magazine fitted through a pistol grip because the catch would be below the grip and the grip would be extremely short. So, it's not a MAC 10/11/etc. or Uzi or a converted pistol.

The length between the catch and the index tabs implies a relatively long magazine well. So it's definitely not a Thompson, but like a Thompson, the magazine catch would be below the magazine well and not through the magazine well.

The magazine appears to be a double-stack single feed and not curved. Odds are it's not a 7.62mm Tokarev, 7.63mm Mauser or other highly tapered round which would usually be curved.

It doesn't have a reinforcement plate at the top, so not a STEN or M3 Greasegun.

There's an upward angle at the top as well. Not an Owen SMG.

We're looking for an older SMG, probably 1930's through 1950's--that isn't a converted assault rifle, that feeds from the bottom with a significant forward magazine well and a magazine release below the magazine well. It's not likely to be a top or side feeder or in a tapered cartridge. there's a lot of space vertically that would be inside the gun and below it, so it's not going to be a pure-tube-like design. We also don't see any round counting holes or other openings as well for the majority of the rounds.

It's likely to be a commercial or smaller military design because it's not a more standardized magazine like the STEN/Sterling family, the MP5 family, or any of the Soviet made guns. It doesn't look like a shop-made or improvised model but it also doesn't seem to have a maker's mark unless it's on the baseplate.

Any chance you can show us the baseplate and/or follower? Maybe tell us which calibers it fits? Looks like 9mm P, but 7.65mm French is also a straight-walled round and would fit in the time era we're looking at...?