Historically there were loads of 'odd' plugs/sockets used in specialist applications where it was deemed necessary to stop random devices being plugged into utility power-outlets whose grounding/voltage/phase/frequency may come as a bit of a surprise to the uneducated.
I posted about a "Hubbell" BS196 plug half-a-decade back - see
https://www.vintage-radio.net/forum/...d.php?p=883422