Brake light switch, Lucas, black button with mounting plate.
Various models:
Triumph T120,TR6 650cc Unit Twins 1971 - 1972)
Triumph TR7,T140 750cc Twins 1973 - 1985
Triumph T150,T160 Trident models 1968 - 1970
BSA Unit Singles B44 etc 1967 - 1970
BSA A65 (1971-73), A75 Rocket 3 1968 - 1975
Norton 750,850cc Commando models 1979- - 031621
Matchless/Norton P11 - 031621
Velocette Thruxton
Part numbers: LU34815, 99-1028, 60-0990, 19-0916, 19-916, 031621, 118SA