That is a rib, but darned if I can tell what that mark is. It is almost certainly a proof mark, but it doesn't look English and I have never seen an American pepperbox with any kind of proof marks (the US has never had a government proof house or a mandatory proof test). Perhaps someone with better eyesight can chip in.
Added: I enlarged and rotated the pic and the mark appears to be a crowned eagle. That could point to Germany, but that gun was made before the European proof marks (with the exception of England) were incorporated into law and standardized, so my books don't show anything like that.
Last edited by Jim K; 12-11-2010 at 08:12 PM..