The model 31 was available from 1931-1949 and was chambered for 12, 16, and 20 gauge.
12ga serial numbers range from 0,000 – 121,000.
16 & 20 ga. serial number are 500,000 – 575,000.
If you are still unsure about the chambering here are the bore diameters and their corresponding gauges:
12ga: .73in or 18.5mm
16ga: .66in or 16.8mm
20ga: .615 or 15.6mm
The choke will may result in a slightly small bore diameter than those posted, but you should be able to tell because the value will be very close, so just get out a ruler or tape measure and try it out.