[Arran]

My guess is that the 46-200 would take some sort of machine screws if they are actually threaded into the Bakelite, maybe something like an 8-32 or a 6-32. As for the type that screw into the chassis the ones I have seen have hexagonal heads and use some sort of sheet metal thread, no idea of what the thread pitch might be. Other then taking the chassis, or a remaining chassis screw, to the hardware store or a dealer in fasteners I don't know how you would find a replacement. The back cover on a wooden cabinet set was likely held on with small round headed wood screws that screwed into some wooden blocks, at least that's how they held it on with my Canadian Philco model 84 (U.S model 50-524) from 1949-50.
Regards
Arran