Good day Mike,
Could the bag be for the Mark V variant of the STEN? It was produced with a wooden stock and pistol grip, and was issued to Paratroops in 1944. Maybe the design of that Mark allowed the butt stock to be detached and stored in the smaller outer pocket of the subj bag. With the word "parachute" on the inside of the flap, I could see it.
My thoughts. Hope it give you another avenue to search.