I believe that virtually all damascus bbls were made of two tubes joined together. The breech section was formed thicker than the rest of the bbl. Many makers tried their best to make this as invisable as possible, but obviously all didn't. I seem to recall Greener stating this "Breech Tube" to be about 9" long but suspect that varied according to exact desired finish contour. The two sections were then sort of "Butt Welded" together.
That joint in the rib will be found on most guns with either a dolls head or top bolt. In these cases a short section of the top rib with the extension is brazed in along with the joining & lug. The remainder of the rib, under-rib etc will then be soft soldered. On guns without a rib extension the entire rib is usually one piece & softsoldered in after the breeches are brazed.
Off hand I can think of no American built gun which used a monoblock, but if someone has knowledge of one I can stand corrected. A few had chopper lumps, but most were conventional or dovetail lump joined.
