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.
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...