I've seen one photo of the Pieper patent (NOT the one you posted, Pete) and it shows a Damascus gun. That COULD be the difference, maybe not, however.
Material, damascus or steel, makes no difference to the proof house, IF it passes, but, it might make a difference at the patent office when one of the materials is a new one, in a new application, which fluid steel would have been-again, I don't know why there exist two patents for the same thing.
I don't have a number for the Darne patent for "Canons Frettes" of 1902, but, I'll work on it.
The last link you listed doesn't open, but, I am aware there were several sliding breech designs that weren't Darnes, or Darne copies. The Levi 4 round double illustrated in The American Rifleman comes to mind.
Best,
Ted
