I do not believe the Imperial Gun in question is made on Pieper's patent breech. Look closely at the picture & you can see signs of the twist pattern under the bluing. It has the knuled ring around the bbls & the rear portion blued but pattern is visable, not a solid steel piece as on the Pieper Breech. Also note there is no joint in the top rib at the juncture. The Pieper breech illustrated with the muzzle ring is the 1881 patent & under this patent the bbls were not screwed into the breech but inserted from the rear & soldered in place. The breech piece stood proud of the bbls, not flush as on those screwed in from the front. My Pieper has the screwed in assembly & the bbls & breech have a flush join & there is no muzzle piece. The US patent # for the 1881 method is #246,195 do you have a # for the 1891 patent?
