Good info above. If it's a smallbore hammerless, then it's just about 100% that it's sideplated, not a sidelock. Easy way to tell for sure is to drop in a couple snap caps and dry fire it. On a sidelock, the axle pin (or screw)--which on a sidelock often has little arrows as cocking indicators--will rotate on each side when you fire that respective barrel. On a sideplated gun, you won't see movement from any of the pins when you dry fire.
What you're probably looking at is a Roma series (or Gamecock Deluxe) sideplated boxlock. IMO, they're very good buys. Prices vary depending on condition and grade, and the 28's are pricier than the 20's and 16's. Range would be from the low to mid teens for a Roma 3 (bottom grade) 20 or 16 in decent shape up to maybe close to $3,000 for a Roma 6 (difference is the amount of engraving) 28ga in near perfect condition. There are higher grades than the Roma 6, but they're very rarely seen.
