The tag price eliminates "graded" or "Custom-built...." guns. The following grades remain: Standard or Field Grade, Tournament Grade, Trap Grade, Skeet, Duck, Deluxe Grade, Magnum. The first place to look is the trigger plate as some will be clearly marked: "Deluxe", "Duck" (pre-1940 3" guns were not marked "Duck"), "Skeet", or "Trap" and Field Grade will have no designation.
Do you know why it's called Model 21 and the first assembled piece is #2167? The guy who assembled the gun that day was 21 years six months and seven days old!
PS. What you probably looking at is Standard Grade. Now the only things left to asses are: originality of wood, cold blue of barrels, position of adjustable screw, and originality of chokes and chambers.

You would not want 3.5" Model 21 that's for sure.