100-300 USD depending on condition.
Sold by Sears Roebuck. Probably manufactured by Stevens.
Manufactured by J Stevens (Savage Arms) in good condition 250 dollars in excellent or better 500 bucks
== == "Riverside" was a brand model shotgun produced by the J. Stevens Co. (which was bought out by Savage in 1920) - These were utility grade double barrel shotguns manufactured between 1877 and 1988. If the serial number is prefixed with a letter it was manufactured after 1949. If there is no letter, it was manufactured prior to that. Thats the best I can do.
The Stevens model 311 was discontinued in the year 1989.You would have to contact Savage/Stevens to get the actual age of your double barrel shotgun.
"Your" Winchester Model 21 Double Barrel Shotgun was manufactured in 1933, giving it an age of 77 years, as of 2010.
If it is side-by-side, probably Stevens.
Hunter Arms Company, Fulton, New York
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You may have to break the shotgun down in to the barrel section and the receiver section first.Look under the barrel section on the flat just ahead of the lug for the serial number.