I have been looking around for a resource that explains, with pictures, the various kinds of third fasteners found on doubles, so far without any luck. The best one I have found was Hallowell but it isn't very complete.Can forum members post pictures and explanations of any third fasteners that they know of? I think this would be a great resource for the site.

I will start it off;

Doll's Head (taken from Hallowellco.com):

Doll's Head - A rib extension on a break-open gun, ending in a circular or semi-circular shape in plan (resembling the head of a doll), mating into a similarly-shaped recess in the top of the receiver, designed to resist the tendency of the barrels to pull away from the standing breech when firing. Because an action's centerpoint of flex when firing is at the base of the standing breech, not at the hingepin, a passive doll's head extension makes an effective extra fastener, even without additional mechanical locks operated by the opening lever.