"Eric 375," Yes, I think the scissors-like mechanism shown in the Abbey patent drawing that appears to operate the bifurcated round bolts that pass through the hole in the rib extension from either side, when combined with the single-bite depicted, do together act as a second-bite mechanically. This is an American-patented form of an early rib extension cross-bolting system. Indeed, it may have influenced other makers with whom the Abbey firm was involved, whether foreign or domestic.
Below is a photograph of a (one of six known)
Charles Daly 200 Grade, Diamond Quality (no black powder or nitro proof marks; undersides of barrels bear the crown-over-crossed pistols trademark of H. A. Lindner) hammer-gun with Lindner's bifurcated cross-bolting system, with each of the bolts passing through a square hole on either side of the rib extension, which in combination with the two under-bites inside the bar, where joined with the barrel lugs, act as the third-bite. This 12-bore hammer-gun was on offer with
James Julia Auctioneers in October of 2015. Remark that the Lindner 'hidden' bolts shown below are rectangular in overall shape (although, I believe, square in cross section) and are not round, as are the bolts (also hidden) depicted in the Abbey patent drawing:
Daryl's knowledgeable remarks above are well worth consideration and also provide intriguing avenues for further study on the matters you have raised.
Regards,
Edwardian
