A 39 Plymouth was a P7 (cheap car) and a P8 (deluxe car).
A 40 Plymouth is a model P10.
A 41 Plymouth is a P11.
A 42 Plymouth was a P14C

A 39 Dodge was a D11 (cheap car) or a D12 (deluxe car).
A 40 was a D14 (cheap car) or a D15 (deluxe car).
A 41 Dodge was a D19, A D20 (Kingsway?)or a D21 deluxe.
A 42 was a D22 or a D23 deluxe.

39 was the introduction (some started in 38) of the independent front suspension across the Mopar car lines. 41 was the last year for separate running boards, in 42 the running boards were part of the body, and that basic body carried on through 48. 42 production ended in Jan of 42 to convert to the war effort.

The P15/D24 web site also covers the prewar years. I've not heard of a specific prewar website. There are a couple great hot rod builds going over on the HAMB board, one with an LA engine installed in a 41 Plymouth, and the other involving a 392 Hemi in a 41 Plymouth coupe. Gene