A good friend of mine bought a Westley Richards Anson and Deely box lock double rifle, top lever Model C leverwork in Cal. 577/500 no.2. Very very fine scroll engraving over the entire action. Serial number in the 13,000 range. Bores are excellent, stock is mint, barrels have been blued, I think I can see some damascus pattern. Top rib says "charge 110 grains Bullet 400 grains". Now here is the question. There are NO under lugs, it only lock up by the top lever bolting into the model c dolls head. It is tight on the face and appears to be little used. Is this bolting system strong? Never seen one without the under pinning. Serial number from Westley Richards book puts in the Best Gun range from 1877 to 1884 build date.
Early Westley Richards guns (and rifles) did not use a Purdey-style underbolt and lugs. The only locking mechanism was the sliding bolt into the rib extension.
Part of the reason for them going to the additional underbolt was the fact that during that time many makers of best guns were advertising the number of bolts that their gun had. Greener had their 'Treble Wedge Fast", Purdey had their "Sextuple Lock", etc.
Evidently, the number of bolting features was a selling point.
The fact is that a properly fitted bolt, placed at the furthest point from the hinge will have the most mechanical advantage in holding the gun closed.