Pete;
I may be totally awash on this as have no actual definitive info, only a small amount of observation to go on. I was however under the impression the difference in the Original Diana & Modified Diana was in the method of joining the tubes to the block. In the original patent the tubes are inserted from the rear of the block which was bored with a slight taper. In this method there is always a step present at forward end of block
On the modified Diana guns the tubes are stepped & inserted into the block from the front & have a smooth transition with the block, though often do have decoration around the joint. The two different construction methods show quite clearly in your pics, incidently. As noted in the 1897 catalog these "Barrels are Screwed in" (from the front). In mine at least the tubes themselves extend only about half way through the block.
Some of the guns pictured in the Sears catalogs though do not carry the name Pieper & simply say "Diana Style". These are the ones I think "Pehaps" may be simply decorated to give that appearence & not be actually so constructed. Again note the bottom of the ad "There are cheap imitations of this gun with the breech ends of barrels ""Painted"" instead of screwed into the breech".
