What is throwing me off is the serial number starts with a M Is M military ???? What year or cubic inch

[Pat McCabe]

Engine stamp

 

[Bryan]

This is what I could find.. Engine Number (t137.com)

[Bryan]

Flathead Six Engine Numbers (t137.com)

[Andydodge]

Plymouth engine numbers should normally start with the model number then the actual engine number, for 1937 it should start as P3 or P4 to indicate a Standard or DeLuxe model then the 6-8 digit engine number............thats why the Forum here is the P15-D24(1946-1948) for both Plymouth and the Dodge variant.....the engine number for all the mopar side valve sixes is as stated in your other thread, above the generator, at the front of the block on a machined flat surface about 3" long and 1/2" high..........andyd 

[keithb7]

What is your block length? We can then get pretty close to a bore size. Stroke is easy to measure. Small pipe plug over #6 cylinder in head.