Approximately 832,000 Model 12's were made from 1909 to 1936. If you post a serial I can date it to year, if it was made in 1921 or later it will be date code stamped on the left side of the barrel right before the receiver. The information on this and how to look up this code for the month/year can be found on the Remington Society of America website. I will add a related link that will take you there.
1918
1912
Made 1912
1918
1910
1912
1920
Remington Works - 1914 (late)
1920
1920
1920
1918