They used a tongue and slot system to join parts and also rivets. Obviously when you set jigs for multiples then everyone will be precise. I am doing two individually but being extra careful to reproduce them with accuracy. I have many many dies for my punch but only one slot punch
but how happy was I that it happened to be the right one
After the parts are made, it is a simple operation of fit and bend the tabs alternately. Anyway it's a good change and it had to be done some time. There is quite a bit of steel threaded mounting block to make and rivet to the ally which is fiddly but very effective.