WW Greener's ran into this problem in recent years when making their new damascus-barreled guns -- plenty of tubes, no ribs to match. Tried to have a damascus knife maker make some but when browned did not match the tubes. Finally located some old ribs unused. Also, they are taking old damascus scrapper barrels, then quartering them lengthwise, then milling out ribs from the resulting quarters. They are many ways to skin a cat in craft gunmaking.