What type of “socket” is this?

MK are a manufacturer of electrical sockets mainly.

To me this looks like an old connector for an electrical storage heater. In my Dad's house the connectors were left on the skirting but an electrician had long ago removed the connection to the fuse board.

I've seen similar looking sockets for very old plug in phones (they used a large version of a headphone plug). I wouldn't assume anything without testing for live wire presence.

That's electrical. It has the old standard of red & black.

Red was live and black was neutral.

I suspect that the house was rewired but the old wires weren't removed but just disconnected (and cut away) at the fuse board. Check if the red (and the black) are live — a cheapo phase tester screwdriver will tell you that.

The reason drury says that is because a lighting phase tester will tell you there is some sort of power present. But a non lighting phase tester doesn't tell you anything so you can't safely assume there is no power present. They're sometimes useful to put small holes in plasterboard.

Oh bloody hell, that's me told! I completely forgot about that possibility!