Keystone was a " Trade Name " used by E. K. Tryon Co. of Pa. The shotgun is listed as being made by Davenport Firearms Co. However Tyron , as other Hardware companies of the era and even today. brought from the lowest bidder, Since the Keystone name was their registered property they could have used it on any firearm they sold, regardless of who manufactured it. Pictures are needed for any one to give you a valid ideal of value.
