Hi,
It looks a bit like the industrial plugs & sockets that used to be made by 'Reyrolle'. The power station I used to work at used them for the 110volt portable power tool system. We also had a miniature version for the 25volt SELV hand lamps.
The 250volt version had a different keyway to avoid the wrong socket being used.
Maybe your house had a domestic version?
Cheers, Pete.