The rising bite you can see in this picture does not exist – at least not according to Boss. The gun is a 12 gauge Boss Over-and-Under. It’s a 1930s era gun, made for 3″ shells and heavy loads.
Even though there are at least two other Boss Over/Unders with the same type of rising bite, there’s no information this feature in Boss’ records. No patents were ever issued for it, either.
Rigby made the rising-bite action famous. They used them on a number of their Best-quality double barrel shotguns and rifles, especially on ones made before the first World War. You can see some examples of how the Rigby-style rising bite works here and here.
On the Boss, the top extension on the rib doesn’t have the horseshoe-style gap you can see in the Rigby. Instead, the extension on the Boss has a gap on its underside. The locking bite rises up into this gap as the gun is closed.