91BFP21 breaks down as follows:
9 - 1969
1 - Chevrolet
B - Full size models (Impala, BelAir, Caprice, etc...)
FP - AM/FM mono radio
21 - Revision level
91BMP21 breaks down as follows:
9 - 1969
1 - Chevrolet
B - Full size models (Impala, BelAir, Caprice, etc...)
MP - FM Multiplex adapter (second part of a two-piece radio)
21 - Revision level
Radio codes
HERE.
Ed