Www.forgottenweapons.com recently did an episode on development of British SMGs, from the perspective of magazines.
It starts with RAF and RN fforts to panic-produce a simplified MP28 Bergman: the heavy, beautifully machined Lanchester Since they had no time for development, they simply copied the MP28’s single-stack magazine exactly.
STEN was a second attempt at a panic-SMG better suited to sub-contracting to smaller shops. Again, panic-production prevented developing a better magazine. Manchester and STEN magazines were inter-changeable. STEN Mark I was overly-complex with poorly-matching parts made by dozens of different shops.
Oh!
Everyone’s Mark I has problems.
OTOH by 1950, any STEN had been “ridden hard and put away wet.”
Britain’s first new-design SMG was the late-war Patchet/Sterling with arguably the best SMG magazine ever!
In a Canada-wank, I proposed a STEN, bull pup, personal defence weapon with a 19” barrel and a magazine that folded to stow parallel to the barrel - like contemporary French SMGs.