Diference between a sleeved gun & a Monobloc is that the breech piece on the monobloc was machined from a "Single" (mono) piece of steel. The breech piece left after cutting off the tubes for sleeving will retain whatever type of joining it had originaly, whether chopper lump, dovetail, conventional, shoe lump or whatever. The act of cutting off the tubes does not convert it to a monobloc, though the method of adding the new tubes is done in a similar fashion.
Henri Pieper's original patent for this method of construction had the "Fretted" construction. This was because the breech piece (Piepers term) was bored on a taper from the rear. The tubes them selves were not turned down but had mating tapers turned on their rear area & were inserted from the breech. Later methods were to turn a stepped portion on the rear of the tubes & insert it from the front with the OD's of both tubes & breech piece matching.
My Pieper hammer double which is marked Modified Diana has the non-fretted construction & the tubes were threaded & screwed into the breech piece from the front. They only extend approximately half way through the breech piece.
