Here is some info:
Viewing a thread - VIN Numbers on 1957 Plymouths
The engine number and model number would have "P-30" but not the serial number. 1957 was the last year for a straight sequential number with no decodable references to year, make, model, assembly plant, etc.
The serial number is attached to a plate on the driver's door hinge post.
The 1957 Plymouth serial number sequences, by plant :
P29 - 6 cylinder
Lynch Road : 14280001 to 14410539
LosAngeles, CA : 25215001 to 25222883
Evansville, IN : 20860001 to 20891720
Newark, DE : 28100001 to 28103737
P30 - 8 cylinder
Lynch Road : 16083001 to 16392956
LosAngeles, CA : 26595001 to 26643618
Evansville, IN : 22330001 to 22450693
Newark, DE : 28525001 to 28534683
P29 serial numbers were for all 6-cylinder Plaza, Savoy, and Belvedere models. P30 serial numbers were for all 8-cylinder Plaza, Savoy, Belvedere and Fury models.
For Canadian (Windsor) production :
P29-1 : Plaza 6 : 95545301 to 95552038
P29-2 : Savoy 6 : 96123501 to 96132216
P29-3 : Belvedere 6 : 95999101 to 95999183
P30-1 : Plaza V8 : 95569801 to 95571764
P30-2 : Savoy V8 : 96188701 to 96197924
P30-3 : Belvedere V8 : 969231601 to 96929547
Convertibles and Fury models were not built in Canada.
American serial numbers also include export Dodge Kingsway and DeSoto Diplomat models built at Lynch Road while the Canadian numbers include export DeSoto Diplomat models.