I did a quick Google search and it seems that M5x1.0 was indeed used for a few very obscure applications like old Peugeot bicycles but is insanely hard to find nowadays. You do encounter weird screws from time to time... a while ago I installed some Italian receptacle boxes (they have listings for all European countries and you can get receptacles to every standard so they're sold in Austria) and needed longer screws in a kitchen with thick tile. The originals look like a very coarse M3.5 and I couldn't find them anywhere. I ended up using M3.5 sheet metal screws.
