The first thing you need to do is determine correct grade; it can't be a "grade 2 (3E)", unless it has been cobbled together with parts from one of each grade. If the gun is an original Grade 2 it will have a single engraved bird on each lock plate (may/may not be in a circle); a Grade 3 will have two birds engraved on each lock plate (may/may not be in a circle). To be sure of grade, cock the gun; then remove the fore arm and swing the barrels off the frame. The grade stamp should be on the water table of the frame and on the barrel flats. Once you know the grade, take it to a couple of collectors for an appraisal (dealers will low-ball if they believe you want to sell the gun). A check of the blue book estimated values will provide a broad general value range; but the amount of original remaining finishes is "everything" as regards determining collector (or highest) value for a vintage double gun.