Follow the instructions in the owner's manual. If you don't have one, you can request one from Remington on their web site.
Model R-12
The stock for this model gun can be found at cabella's
The RemingtonModel 121 fieldmaster was introduce in 1936 and discontinued in 1954
Depends on condition, condition, and condition, from just good to excellent 500-1000.00
Check the date code stamp in related links.
March 1940
You can request an owner's manual for free at the Remington website.
check the date code stamped on the barrel. Remington Society of America, Manufactured Date link has the details.
Go to www.e-gunparts.com/model.asp?idDept=204 very good diagram
June, 1948
Check date code on barrel - see related links
This serial number was produced between 1936 and 1954 out of a total of 199,891 Model 121s produced.