Identifying a (power?) socket
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.