One new Stromberg brand 150-CD series carburetor diaphragm, part number 131-8000, DC200PP, B16001, or 366-670.  This fits:

Triumph Spitfire 1970 - 1980 (150CD carbs)
MG Midget 1500 1975 - 1979 (150CD carbs)
Sunbeam Alpine 1966 - 1967 (150CD carbs)
Triumph GT6 1966 - 1973