Regarding the Sten trigger mech cover. There were loads of manufacturers of this piece and there were at least 3 variants of it. Screw holes, locating dimples and screw-holes AND dimples plus the various manufacturers types. Manufacturers were given the spec and manufacturing detail. But they were allowed to make representations to the MoS for relaxations to parts of this in order to make the best use of their facilities, material in hand and costs. So there were fully pressed covers, folded and welded as shown in the Sten book, punched, folded with ears spot welded etc etc.
You could write another book on just the covers if you were a real rivet counter type. Indeed, I relate the story in the book of how one small company in Wiltshire punched them out by the thousands