Although not called "Mono-block" at the time as I recall Henri Pieper first patented the process of using a solid steel breech piece with tubes inserted. His patent referenced barrels inserted from the breech & there was a step down at the end of the breech piece, which is often referred to as "Fretted Barrels". Most, if not all, of these Pieper guns, were marked, Diana. I seem to recall that some of the French Robust guns used this system.
I have a Pieper hemmer gun which has the tubes inserted from the front, without a step. On these barrels the tubes go about halfway through the chamber & re both threaded & soldered. The breech piece is steel & consists of the hinge hook, double underbites & a doll's head, all one piece. The tubes are twist. The seam is visible & is in fact marked, which has never bothered me in the slightest.
The Pieper system was patented in the US on Aug 23, 1881, under ptent #246195 so this system is anything but new. I do not know for certain but highly suspect that Beretta coined the term "Mono-Block"
