Production started in 1955 and continues....
The date (month/year) can be determined by the data code stamped on the barrel.
A diagram of the date code location and a table explaining it can be found the the Remington Society of America's (collector site) in the manufacture date link.
http://www.remingtonsociety.com
it is below the feildmaster logo
OH on the barrel? that's not the serial. If it's early probably July 1961.
Check the barrel codes on the Remington Society of America. http://remingtonsociety.com/rsa/questions/barrelcodes
Check date code, 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
no. the serial # will normally be 5 digits.
1946, it will be date code (month/year - letters) on the left side of the barrel. You can look this up under the manufactured date on the Remington Society of America site.
You need to look up the barrel codes, the details are on the Remington Society of America here: http://remingtonsociety.com/rsa/questions/barrelcodes
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
50-150 USD or so
All 121's are date code, see related links for the table/information for looking it up.