The first digit(s) in the serial number will tell you where it was made.
1.Flint; 2. Tarrytown; 3. St. Louis; 5. Kansas City; 6. Oakland; 8. Atlanta; 9. Norwood;, 12. Buffalo; 14. Baltimore; 20. Los Angeles; 21. Janesville. Numbers not shown weren't used.
Next letter will tell model year, in your case there will be a J for 1951.
Next is the series ID, P for a 3100.
Next is a letter for the month it was built. A thru L for the month (J=Oct).
The last digits will be the production number.
