The extension rib is the "third fastener", and the part that constitutes the screw grip. As the top lever rotates toward the closed position, a large screw rotates with the top lever and pushes down on the rib extension as it rotates to insure a tight lockup. Of course the two underlugs are the basis of the locking mechanism, but the Webley screw grip adds a bit of class to an otherwise simple action. Several makers used the Webley patent on some of their guns. I won't try to list them all. Some were top of the line full sidelocks, stocked to the fences, like my Evans pair. Some were simple, unadorned boxlocks, like my old W&C Scott 16 gauge.
