"I'm not quite happy with the assertion that the STEN was designed to take MP40 magazines "

Was not the Lanchester a "highest form of flattery" of the Germanicon MP-28, itself an "upgrade" of the old MP-18?

If this is the case, the Lanchester was "designed" to take a clone of the MP-28 magazine, not the MP-38, which came ten years later.

Furthermore, if the STEN were required to take magazines "already in production" that could only mean the Lanchester / MP-28 type.

Anyone have ALL of the various mags (and / or guns) close to hand?