[Richard_FM]

Thanks for all the feedback.

I remember one of the classrooms at my primary school had an odd socket on the wall with 2 holes in.

I assumed this was a redundant 5 amp socket, though all the other sockets were standard 13 amp ones. All the wiring was in conduit pipes rather than plastered into the wall, with this socket being on a separate conduit & looking long disused with many layers of paint on.

The school was built in the early 1950s, & the above talk had me thinking it might have been used for a radio system originally.