The No. 4 Featherweigh L.C. Smith in Houchins book has a concave rib as does the regular frame No. 4 in Brophy's book. While I certainly wouldn't make so bold as to say flat ribs don't exist, I thumbed through all the pictures of pre-1913 guns in Houchins and they all had concave ribs. In Brophy there were a couple I really couldn't tell.