Perhaps BSA Roger, though just about any basically equipped small engineering works or car garage workshop for that matter, could carry out the simple task I suppose.
I can picture a ****ed off 16 year old apprentice and a huge stack of magazines, that's for sure! It certainly isn't done with any great care, just a roughly machined off rib and a quick squirt with suncorite...
100,000 was probably enough to ensure you didn't have a backlog at the factories of completed No4's waiting for magazines....
I wonder if it was actually just a pre emptive measure, rather than an actual requirement?
Update: Looking at my example, it doesn't look like it's ever been fitted to a rifle in its modified form, no typical tell tale signs of mag well markings or cartridge marks on the platform...
How about yours Roger?