Here are a couple of other pics that show some detail of the "rising bite" third fastener. These are courtesy of Roosevelt & Drake's website (lifted).

To explain the function: when the operating lever is thumbed to the right, the conventional Purdy underbolts slide back, and the bolt at the rear of the loop extension of the barrel rib (the rising bite) slides down into the action. This gives clearance for the lumps and the loop third fastener to move free of the bolts. Thus, the barrels can be tilted open.

Clear as mud, eh??

Best,
Curl