In Pieper's original patent the bbls were not threaded in to the breech piece (mono-block). The breech piece had tapered holes through it & the bbls were inserted from the rear & seated in the tapers. There was thus a step down at the forward end of the breech piece, or what we generally refer to today as "Fretted". The tubes being turned down & threaded in to the breech piece & giving a smooth joint came at some later point. I do not know who, but suspect someone patented this method.
One of the big "Bragging Points" of the mono-block was it did not require brazing heat to assemble, but was a combination of mechanicl & soft solder, but had far greater strength than a normal assembly would if done with only soft solder. Conventional, Dovetail, Chopper & Shoe lumps all require brazing heats for maximum joint strength. The dovetail chopper lump as on the M21 & some others does not require brazing as the two are mechanically held together & pinned to prevent shifting. They can thus be soft soldered to complete the joint with no fear of bbl seperation.
Note that all of this has only to do with the strength of the connection between the two bbls & does not pertain to strength as regards to handling pressure of firing nor the connection of ribs etc.
Pieper did use a band at the muzzle in his oroginal patent, to say it was a "Requirement" is I think stretching the point. The Pieper hammer gun I have built with the later method of screwing in the bbls does not have a muzzle band. Also on mine the bbls have short stubs which extend only about half way through the breech piece, not all the way as on those pictured. He seems to have used several methods over the years. Mine is marked Modified Dianna.
A 1911 "Alfa" catalog still lists Pieper guns made on the mono-block principal, though they are simply referrd to as having a one-piece steel breech piece. I so far have never seen a reference to Pieper using the term mono-block.
