Go to www.e-gunparts.com/model.asp?idDept=204 very good diagram
The RemingtonModel 121 fieldmaster was introduce in 1936 and discontinued in 1954
Follow the instructions in the owner's manual. If you don't have one, you can request one from Remington on their web site.
Photocopies can be found on ebay.
Start off by contacting Remington. They are good about keeping parts available for their products. If that fails, go to Gunpartscorp.com. Their selection is impressive. Jack First is another contact. There are sometimes parts available from eBay and auctionarms.com.
All 121's are date code, see related links for the table/information for looking it up.
The stock for this model gun can be found at cabella's
The serial number of Remington rifles will not give you the date of manufacture. The two or three letter code stamped on the base of the barrel will tell the year and usually also the month of manufacture. On a Remington 121 this letter code OS usually on the left side of the barrel right next to the front of the action. I have a 121 with the letter code "OF". This indicates that my gun was made in July of 1937.
3 numbers are factors of 121. They are 1, 11 and 121 itself.
Model R-12
Check the date code on the barrel. See related links for information on the Remington Society of America.
check the date code stamped on the barrel. Remington Society of America, Manufactured Date link has the details.