For '71, Plymouth split the coupe and 4-dr. They were two completely different cars. Prior to '71 ('67-'70) all Plymouth B-Bodies were part of the Belvedere family. This is from '68...
L = Low - Belvedere
M = Medium - Road Runner
H = High - Satellite
P = Premium - Sport Satellite
S = Special - GTX
The first letter of the VIN on each of these cars is 'R'. The second digit is the letter listed above depending on what car you have. As a coupe in 1971, the M, H, P, and S apply. The E (economy) and L was used on the 4-doors (cabs and cop cars).