Years ago I had 4 different Stens. 3 Mk II's and a Mk III. Along the way, one of the Sarco-type companies had a deal on Sten mags, I think it was like 50 for 35.00 bucks or something. Anyway, I bit the bullet and bought a large swack of them.
Turned out, only about 10 actually worked in all 4 Stens. Others would work in some Stens but not others. About 15 of them were real turkeys and I flogged them off at a buck a piece in a gunshow.
I had access to undeclarable amounts of 9 m.m. ammo at the time, and could actually spend my days off firing the damned things to see which mags worked with which.
The Stens and their mags (and all the guns I owned back up there) are long gone now but I can tell you this; you GOTTA make sure the mag you have WORKS in the Sten you have. When they work, they work fine. When they don't work...well, they don't work so fine.
And your Obergruppenfuehrer target might just go driving off into the sunset because of it.
Photo; Just laying there on the basement floor in this old photo of some of the stuff my Dad and I had are a Sten Mk II and a Sten Mk III -- amongst other things.