The photo below shows a front view of the threaded spindle that moves the third fastener up and down to lock on the stepped rib extension. This is from a Woodward marked W&S Proprietary action, a widely used action.
The stepped rib extension is shown above the rear Purdey double bite in this photo.
Rocketman:
Well, I kinda, sorta agree.
The threads on the spindle don't engage the rib extension in any way, and therefore are not what fasten it. In order for the threads on the spindle to engage the barrel extension, the extension would also have to be threaded, which it is not.
The beveled TOP of the portion of the rib extension that extends beyond the doll's head (straight extension rather than doll's head on the Proprietary Hammerless Model) that is visible in the lower photo, engages a mating bevel on the BOTTOM of the rotary locking lug on the spindle (visible in the upper photo, just above the threads) - creating an interrupted screw thread interface on these actions. As the action closes, the top-lever/spindle rotates clockwise, tightening the "screw" interface formed by the angled surfaces on the rib extension and lug, essentially "screwing" the top extension down into the mortise in the standing breech. That's exactly how a Jones screw grip action fastens the barrels to it's action, and it's also how a screw grip fore-end fastener fastens the fore-end to the barrel loop. That's what a screw grip is. It's so named because it's a rotary lock that tightens an interrupted thread interface, and it had that name long before Webley & Brain came along.
