[paul2748]

Barry's correct

The fist letter was always the decade, starting with B for the 50's, although it was not used for the early 50's. Around 57-58 or so they started using the letter for the decade for replacement parts and new oem parts using the first number for the year (6 for 56) so B6 was 1956

Part numbers and engineering (casting numbers) used this method until 1997 per the answer above