This a  Brake Light Switch, Thread size 1/8x 27 NPTF. Austin Mini 1959 to 1976, Jaguar E-Type 1961 to 1971, MGB 1963 to 1967, Triumph TR3 from TS60001, TR4 to CT26929, Sprite, Midget 1958 to 1967, Mini to 1976