So in that Context Pete, it would seem like the gun was designed around the mag. & not a mag to fit the gun!... usually, it's the other way around!I cant imagine that happening with something like the EM2. Have you seen the Mags for them? I wouldn't want to try to make one!
The fact that GermanMp.40 Mags fitted & used in an 'Emergency', is purely coincidental then? But work they do, in examples I have tried & fired in the past. When at the time, Sten mags were not to hand.