A good bit more "swamped" than even most swamped ribs. Assume there is a short, raised rib at the muzzle as well?

The Helice bolting system was invented by the French gunmaker Verney-Carron back in the 19th century. Time tested and reliable. V-C is still in business today. When the patent ran out, other French gunmakers began using the Helice system and so marking their guns. In many cases, Helice (which means screw or spiral) is the only "name" you'll find.