Before the Gun Control Act of 1968, firearms manufacturers were not required to serialize guns. I speculate that many opted to leave s/n's off in an effort to keep the prices down, by eliminating a costly manufacturing step during the production of certain firearms.
Not easy to pin down the YOM on some of these non-numbered rifles. The value will depend on condition, and you can look around on the auction sites like auctionarms.com or gunbroker.com to see what they are actually selling for. SW
I am really not an expert - distinguished or otherwise!!