Image 20240318-110718 hosted in ImgBB
ibb.co
It's a 15A round-pin.
The smaller version is often used as a connection to your lighting circuit. Things like bedroom lights and so on, where they are connected to a low amperage circuit.
I am not aware of the 15A being used in the home, it's more of an industrial lighting circuit thing. Perhaps there is a specific reason why one was added to your fuse box. Something someone needed more than 13A for. I would check the fuse on it in the fuse box and potentially downgrade it to a 13A socket and smaller fuse (breaker). It will be more use to you.
(In other words, don't buy an adapter, replace the faceplate and the breaker).