BS 546, "Two-pole and earthing-pin plugs, socket-outlets and socket-outlet adaptors for AC (50-60 Hz) circuits up to 250V" is a British Standard for three pin AC power plugs and sockets. Originally published in April 1934, it was updated by a 1950 edition which is still current, with eight amendments up to 1999. BS 546 is also the precursor of current Indian and South African plug standards. This plug is also used in Singapore to a limited extent. Although BS 546 plugs and sockets are still permitted in the U.K. and are used for special purposes, new installations since 1947 generally have used BS 1363 sockets and fused plugs.
Read more about BS 546: History, Applications, Characteristics of BS 546 Three-pin Plugs, South African Derivatives