One new old stock original Lucas 6GC model voltage regulator, part number 37478, 37187 or 37307.  This fits:

Armstrong Siddeley Sapphire 234 & 236 1956 - 1958

Bristol 405 1956 - 1958

Bristol 406 1958 - 1960

Citroen DS19 with Lucas equipment 1957 - 1960

Citroen ID19 with Lucas equipment 1959 - 1960

Lotus Elite 1959 - 1960

Vauxhall Victor 1958 - 1960

Vauxhall Envoy 1960

Vauxhall Wyvern 1955 - 1957

Vauxhall Velox & Cresta 1955 - 1960