Green’s Dictionary of Slang

1. (Aus.) an M.G.B., the ‘B’ model of the M.G. sports car.

1960		A. Buzo Front Room Boys Scene i: Reckons [...] the old man’s going to cough up for a B.
1969		A. Buzo Rooted I iii: Let’s hop in the B and fang up to the beach.

2. (US black) a Cadillac Brougham.

1980

E. Folb Runnin’ Down Some Lines 84: By far the most popular car among those I met was the Cadillac – variously referred to in the vernacular [...] by special model names, the B (Brougham).