A couple comments: the condition of the metal and screws says this gun has been cared for, or at least not messed with by Bubba. Only the screw on the cover plate for the flip-up buckhorn sight appears to have any degree of buggering. A lot of these guns wind up getting attention from Bubba's Badly-fitting screwdrivers and suffering for it.
The heavy overcoat of poly is a detraction, but OTOH it also has likely protected the wood against whatever may have come along and it can be removed if you feel like going the restore/refinish route.
The silver "engraving" on the rib is actually an inlay. Sometimes it's in silver, sometimes in other metal. It's almost certainly the name of the retailer. "Coln" is an alternative form - no longer really in use - for the name of the city we call Cologne.
The paradigm in those days was that retailers would contract with makers, usually in Suhl or Zella-Mehlis, sometimes Ferlach, for them to make a certain number of a certain model of gun. The work would be broken out among specialists in those towns who would make the barrels, the stocks, etc. When complete the gun would be delivered to the retailer with those workers' marks stamped on the insides - for liability purposes - but with the retailer's name usually on the rib. Any combination of options - the fishtail lever, the flip-up peep sight, wood quality, stock dimensions, the type, complexity and coverage of engraving and checkering, traps in the buttstock for rifle ammo, etc., could be specified. Anything for the right price.
The extension, which you appear to have [had] removed was likely not original. Keep in mind that 100 years or so ago people were smaller, the product of lower nutrition levels in those days. A man 6 feet tall stood out in a crowd. So, stock dimensions would reflect that. If you want to have it extended, you could go with either wood or maybe buffalo horn [one of the members on this site occasionally has some for sale] between the metal butt plate and the existing stock instead of what looked like a late-60s vintage white-line and rubber extension. Won't be cheap, but it will be nice.
I think the keyed fore-end says 1880s, but that's just my opinion.
Value? Hard to say. You could not get the engraving duplicated today for less than a couple times what the retail value of this gun, used, would be. And the metal work shows a lot of high-skill, high-quality hand work. I think Sharps' estimate is probably good, but the market for these is slim and you might wait a long time to sell, even at a fair price.
