I have a Keystone Arms 20 gauge single barrel. I have found next to no info on line regarding this gun. I got it as a project gun for the summer to do with my seven year old son. I would love to be able to teach him the history of the gun. Sereial #A547xxx
But it comes under the Keystone, 20 gauge and single barrel;
Otherwise, if you have something very old, it is mot likely a "hardware" store gun made by one of the old gun makers and branded for a particular retailer
Guns with the name Keystone were made by many firms. Among them are Crescent Arms, WH Davenport, H&D Folsom and some were Belgian made. It could be ok but it should be checked over before using it.