This 2.7cm long fragment is from the Brigantian (U. Mississippian) of SW Scotland. I've skimmed through the Treatise (orthocerida and actinocerida) and a few other books but can't find anything Carboniferous with this sort of decoration.
There's no siphuncle visible at either end so I guess it's a piece of the living chamber - a pity because it might have helped narrow it down.
Any comments appreciated!