First, I believe that the "corn cob" fore end would indicate post-war. Pre-war fore ends are small checkered.
The 30's and early 40's have a smooth finish on the top of the receiver while post-war has the rough (phosphate like) finish.
Serial number is on the front face of the receiver. I'm not aware of the serial number anywhere else.
I'm only able to reference 20 gauge guns that are 1941, 1946 and 1949 vintage.
You can verify the age using the serial number.
