Plymouth Model Numbers.

1928 Model Q
1929 Model U
1930 Model 30U
1931-32 Model PA
1932 Model PB
1933 PC
1933 PCXX
1933 PD Deluxe
1934 PE Deluxe
1934 PF Six
1934 PF Special Six
1934 PG Business
1935 PJ – code was used on both Business & Deluxe models
1936 P1 Business
1936 P2 Deluxe
1937 P3 Business
1937 P4 Deluxe
1938 P5 Business/Roadking
1938 P6 Deluxe
1939 P7 Roadking
1939 P8 Deluxe
1940 P9 Roadking
1940 P10 Deluxe
1941 P11 “Plymouth”
1941 P11D Deluxe
1941 P12 Special Deluxe
1942 P14S Deluxe
1942 P14C Special Deluxe
1946-48 (Early 49) P15S Deluxe
1946-48 (Early 49) P15C Special Deluxe
1949 P17 Deluxe (111″ wheelbase)
1949 P18 Deluxe (118.5″ wheelbase)
1949 P18 Special Deluxe
1950 P19 Deluxe (111″ wb)
1950 P20 Deluxe (118.5″ wb)
1950 P20 Special Deluxe

Model names came in to use in 1951
In MOST years, the engine number started with the same model designation. For some reason, in 1937, all engines had a P4 designation, even in P3 Business models. Likewise, there were no P17 or P19 engine codes in 1949-50. The 1942 and 1946-48 engine codes began with either P14 or P15, with no “C” or “S” designation.
Model information supplied by Jim Benjaminson