I have seen this double 3 years ago in Wagner's workshop. it was pretty rusty standing before in a cellar that was flood once. dont had so much time but we talk a little bit about it and Wagner had the opinion that this was a unique british cartridge and had nothing to do with the german bore cartridges. definitely a cartridge thats absent in Hoyem British Sporting Rifle cartridges.
Wagner is a very knowledgeable dealer I have buy 8 guns, rifles and pistols over the years.
seems to be he have found the time to restore it now to his former glory.

rest of the pics before they gone