Before Din-rails were invented all our panels were custom-made. They were usually surface-mount or steel boxes without a back and threads to mount rails on. There were metal (now plastic) rails the Diazed elemnts and other stuff were mounted, and with a surface-mount assembly you could cut these rails just as long as you wanted to. They're stiil on sale, because metal ones have to be replaced when any work is done. The diazed elements were linked with flat brass straps with holes that fit under the screws of the element.
Din-rail panels are a plastic box with one or more rails, neutral and ground bus bar in it. (Neutral commonly unused, as 2 pole breakers are used) Insulated fork bus bars are available, but the norm are short wire links from load side of main GFI (on the rail as well) and the breakers.
4 pole GFIs cost 4 euro less than 2 pole ones (no joke). Each and every service here is 3ph, just sometimes 2 of the phases are terminated at the main fuses or all 3 taken to the meter.
