I don't know about Monk's guns, but Pieper made his guns by the Piles. IOnhis original 1881 patent the tubes were inserted into the one piece steel breech piece from the Rear & thus had the step at the end of the breech piece. These guns are generally marked Original Diana. As Drew stated later guns marked Modified Diana had the tube threaded in from the front & only extended about halfway through the breech piece.
Although I do not believe Pieper ever used the term Monoblock both versions were of that description. I am not sure just who Coined the term Mono-Block, but highly suspect it was Berretta.
I have an old Pieper hammer gun in a rather poor condition marked Modified Diana with the tubes installed from the front. these guns do not have the step. Mine has twist barrels with the steel Breech Piece.
