Monotype in Surrey were the assemblers and a principle producer of parts. Mind you, with the advent of the dispersal Mk2 guns, the parts were made as far North as Gateshead and as far South at Cambourne in deepest darkest Dorset. But while these places made 'Mk2' parts, remember that this was a dispersal scheme - along the other weapon lines. So if butts were in short supply at Enfield, they called in Mk2 butts from Tibbelhams. Likewise, if they were short of Mk1 pistons and pistonb extension assemblies, then they'd come from Gateshead.
So when the real experts show you an all singing all dancing amk1 gun from, say, 1942/3 the truth is that it could have come fitted with all manner of parts. The crunchie fighting in Italy didn't give a fig where the parts for his Mk1 or 2 gun came from. Just so long as it worked.
I mention Italy because a few weeks ago I was in correspondence with an Italianbattlefield recoveryist (- for want of a better word. and VERY interesting fellow too). They had recovered a 1943 Enfield Mk1 Bren. Well, it was a 1943 Mk1 Bren body but literally everything attached to or inside it was Mk2 - including the butt slide and butt, piston and barrels too!
Sorry to go off at yet another tangent