These plugs and socket are alternatives to the BS1363 fused plug. They are all designed to be installed on a 30A ring circuit, and like the BS1363 type all contain a fuse, have three pins, and are rated to 13A maximum.
None of these are in use any more - ultimately the BS1363 flat pin model was used everywhere, at the expense of these ones.
At the time, the circuit designed to be used with these was called a 'ring main'. The correct name today is ring final circuit, however ring main is still used by many people.
Dorman Smith fused plug and socket
Dorman Smith fused 'ring main' plug with the fuse as the live pin, alternative to BS1363.
Wylex Fused Ring Main and unfused variants
Wylex fused 'ring main' plugs and sockets plus similar non-fused variants for 15A circuits.