This is a vintage bayonet socket, very commonly used in the United Kingdom with bayonet base bulbs. This one came included with a B.T.H. Carbon light bulb that I purchased.

The bayonet mount is the standard light bulb fitting in the United Kingdom and in many countries that were members of the British Empire including Pakistan, Australia, Hong Kong, Fiji, India, Sri Lanka, Ireland, and New Zealand, parts of the Middle East and Africa and, historically, France and Greece. (Wikipedia)