Noticed this plug on my wall from when I moved into this house, and I just started wondering again about what it’s actually for. This is in the UK, if that helps. It’s on a big bulky box hanging on the wall below my desk next to two regular plug sockets.
Edit: best suggestion I’ve seen here is that it could be a fuse box for an alarm system. Makes sense since this house did have several security systems before I moved in. Also, for added context, this is in a bedroom and the wire coming out of it goes straight into the wall.
This is fuse box, with a flathead you should be able to open it and/or pull a small tray with the fuse itself inside. You Brits do your electrical wiring with ring-shaped circuits and put a fuse in every outlet.
This kind of outlet is intended for things you don’t unplug and a socket doesn’t make sense, usually boilers, ovens, stoves, ACs, alarm systems as already commented…