The above contributors who suggest Replacement barrels by Ford as the most probable scenario are, I'm sure correct. If you go to Bagnall and Kirkwood ( Google Our History- Bagnall & Kirkwood ) There is a complete list showing the first serial No of each year Pape's guns were made. From this establish date of manufacture. Then post a photo of The Ford barrel Proof marks, if not practical then a full description of them should be sufficient to establish the era of proof. Birmingham Proof marks were date coded after @1925. This is indicated in an Inverted Y within a circle approx 5/16" dia. Let us know and I'll give you some background on Ford after Coffee and Breakfast!
Good Luck
