I have seen Westley Richards guns with only a "third" fastener
and just came upon an article of Ross Seyfreid's in an issue
of DGJ which mentions that this 10ga. gun is still tight after
125 years' use. Picture below.
Makes me wonder in which plane the forces of firing a double
are actually in. With all of the variables such as fasteners,
bar length, hinge pin location etc. it seems very difficult to say
which design is the best. This single lump, single fastener
system would seem woefully inadequate today, but I suggest WR
knew what they were doing. Although, as mentioned above, double
rifles and shotguns behave very differently.