its a shallow box but set in deep remove them connectors loads of depth to fit a single socketAre you in the UK?
The first thing is to look at it, the location of it, the size of the cable and the depth of the box will all give clues as to what this was originally for.
Looking at the picture I see things which suggest it is likely to have been a lightswitch.
Then after that use a voltage tester (not a neon screwdriver or anything like that, they are not safe to be poking in mains wiring) and operate circuit breakers one at a time to establish which circuit it is connected to.