The value of a model 121 is greatly determined by condition and variation, for a general 121A (standard grade).
Mint (100%) $700-800
Excellent (98%) 600-700
Very good (95%) 500-600
? (90%) 400-500
Below it's a shooter from 100-400 depending on overall condition of blue, wood, and rifling
The RemingtonModel 121 fieldmaster was introduce in 1936 and discontinued in 1954
Winchester didn't make it, Remington did. Worth 222-350 depending on condition and lust level of buyer
Check date code on barrel - see related links
June, 1948
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.
Manufactured: 1947, value dependent on condition. I bought a model 121 fieldmaster brand new in 1954 and they were supposed to be the first hammerless pump . I do not know what year they originated but , now you know it was at least in 1954 or before . Wish I could pinpoint it better for you but you now have a jumping in point.
All 121's are date code, see related links for the table/information for looking it up.
no. the serial # will normally be 5 digits.
June, 1941
1949 - there is a date code on the barrel for month/year details on the Remington Society of Amerca here: http://remingtonsociety.com/rsa/questions/barrelcodes
It says it on the barrel, otherwise if there's no riffling it's been modified or shot out, but quite unlikely.
Send me your email & I'll send you a scan of the manual. If you want to purchase an original manual you can try eBay or gunbroker.com but most of those are just copies of an original.