Note in the pics Pete supplied that on the "Original Diana" there is a decided step down to the tube at forward end of breech piece (mono-block) while there is no such step on the "Modified Diana". Pieper's original patent called for the breech piece to be bored on a slight internal taper, with the tubes having a matching taper being inserted from the rear, thus the step down. The chamber was thus cut entirely in the interior of the tube.
On the Modified Diana design the breech piece was bored & threaded "part way" through from the front. The tube was turned & threaded to match with a shoulder to butt up against the front of the breech piece & then screwed in from the front. The chamber was thus cut partially in the breech piece & partially in the tube with the od of the tube shoulder blending with the breech piece.
Nota Bene (note well) the steel breech piece in the patent drawing. It is "All" machined from "One(1)" piece of steel. As far as I am aware of Pieper never used the term "Mono-Block" calling it simply his one piece steel breech, but that is the derivative of the word. Cutting off the breech section of a bbl assembly consisting of 3-4 parts brazed together "Does Not a Mono-Block Make". If you don't like the term Sleeving simply call it re-tubing, but it is not a mono-block unless it was originally machined as one piece.
