As far a forming mag. housing boxes is concerned...
There were various methods used in Mk. II parts production. No matter if components were made in a converted garage or by an experienced metal fabricating firm, industrial tooling would have been used. Whether a flypress was used with a die, or a pressbrake used to fold a strip into a box, the toolmarks that would be left would reflect permanent tooling, and jig welding. The cuts for the magazine catch would likely have been made when the blank for the box was punched out. This requires a two stage stamping operation, pierce the holes first, then punch out the blank. No contractor would be making mag. housing boxes by hammering sheet steel around a forming plug held in a vise, or make the cuts for the magazine catch by chain drilling, cold chiselling and filing, or with an end mill. Dimensional standards could not be maintained, and the quantity of parts produced would be a joke. A one off box made in a home workshop is going to show evidence of hammering, filing, adjusting, etc. Operations to remove these marks would leave their own marks. Late Long Branch Mk. II housings are perhaps the most sophisticated - the collar and box are formed from a single piece of sheet, with minimal welding. Lines Brothers was an established, experienced manufacturing concern. They may have changed their processes, if a better, more efficient, more cost effective way of making parts was developed, but unless this happened, parts would have been made on the same tools using the same pattern of jigs and dies, with thousands upon thousands of indentical parts being turned out. I have observed variations in the spot welding pattern used to attach mag. housings to Mk. III carbine casings. Different shaped electrodes were used, so it was not just a different operator. Unlike Mk. II guns, Mk. IIIs are very consistant.
Laidler lists the Sten parts subcontractors. The more sophisticated parts tended to be made by established metalworking concerns.