Manufactured 1940.
They month and year can be determined by the barrel code, all the details can be found on the Remington Society of America here:
http://remingtonsociety.com/rsa/questions/barrelcodes
This serial number was produced between 1936 and 1954 out of a total of 199,891 Model 121s produced.
March 1940
Check the date code stamp in related links.
Check date code on barrel - see related links
When was the serial? They all have serials. They were made from 1936-54 and have date code stamps on the barrel which can be looked up on the manufacture date link on the Remington Society of America.
Check date code on barrel, see related links
check the date code stamped on the barrel. Remington Society of America, Manufactured Date link has the details.
1950, there is month/year date code stamped on the barrel, you can look up the code on the Remington Society or America.
Is there a question here? All 121's are marked with a serial on the left side of the frame.Made about 200,000 from 1936-1956, date code stamped on the barrel if it's original or serial list/date is also known.
Don't have 121 serial list, check date code stamped on barrel. See related links.
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.
Probably VERY early 1939. Best to check the barrel codes, look up the details on the manufacturing date on the Remington Society of America.