Bulldog:

Purdey double underlugs alone are sufficient for a double rifle, provided everything is fitted properly, which it rarely is on the cheaper guns. A rib extension (straight, dolls head, etc.) is highly desireable as it helps keep the hook square on the hinge. Sadly, the cheaper guns are the least likely to have one, and the most likely to need it. Clipped fences, as seen on the Manton above (looks to have been made for Manton by F. W. Heym) also help with this.

Once there is some wear on the locking surfaces, the barrels get a bit of a running start when they try to hinge open on firing, and looseness will then increase rapidly due to the battering effect. The various "third fasteners" that engage the rib extension are intended to prevent this, keeping the gun on-face longer. The best are the Webley Screw Grips seen above (A. & W. C. on the left and PHV-1 on the right). Screw grips are particularly effective because the engagement wears in rather than wearing out. I'm entirely ambivalent about the other common types. Not to be contrarian, but I think I've probably seen more double guns with Greener cross-bolts off-face than those with any other type of third fastener. Holland's hidden type (like the Royal) seems particularly ineffective as well.
------------------------------------------------