under-hammer pistol, cal. .50 percussion smoothbore; the grooves are for a device to screw in or out the barrels for loading. Over and under / superposed pair of round 3" barrels,  fine engraved receiver, barrels of brownish color, plain wooden grip. Two pistons, one for each barrel. The screw behind the trigger is an uncommon safety. There are no markings, numbers or proof stamps, and one can assume, it is American made.
A rare pistol in fine condition    $2,750.