I have one of them in my house on the first floor.
Does any one know what this plug is for?
It is a telephone socket.
Looks like a very very old phone socket. There was even a newer version of that before they went digital, I think.
Geeze, can a mod cut that image down to size please? What happened to the 800x600 limit?
A so called Reichle De-Massari a.k.a TT87 / TT89 telephone socket/plug. For example used to connect a DSL modem if you have only fiber to the curb.
There are adapters/cables from TT87 to RJ11 and RJ45.