We found this socket in a house in Belgium of the 50's. There are no wires connected, and there seems to be no numbers or identification on it.
Does anybody know what it is?
Google really did not want to help, because any such queries were glutted by travel adapters with all the different socket codes (A through J). Americans may note those cover only US NEMA 1 and 5, and don't cover NEMA 7, 15, L5, 23 and many others intended for heavy-duty use.
That is the situation here. This socket is for heavy-duty loads.
It is not terribly unlike those pictured here in pictures 17-20. This one has a different keying, so is for a different phase relationship or perhaps different current/fusing.
It looks like a Type I socket, but it's listed for APAC and not Europe: