I think that you're right BAR. Some were made from round stock, like bayonet scabbards are. First cut to length then semi rolled to an oval. Put onto a former and pressed downwards into a shaped housing to form the flat front and parallel sides. Then a roller comes in and rolls the rear reinforce/corrugation we see in photo 3. Former removed and there you have it, a magazine casing. idn't I mention this in the little booklet? There were as many variants as makers who were left to their best devices in order to make the best use of their facilities.

Don't forget also, that the M31 will probably indicate the maker of THAT part as opposed to the whole assembly