Well I think my explanation is more too the building of this gun , English gun making from 1800 had a fair number of patents registered at the patent office that greatly increased through the percussion and into the breech loader period. Lets not forget that most types of ignition continued well into other periods if we take the flintlock many were still been made in the 1830 plus the same with the tube lock which some were still made in the 1850 especially for the wildfowler also percussion sxs shotguns still been built in the 1910 for the market also Belgian pinfires till 1930s.Maybe you are correct , Caplock shotguns started being used in England about 1835 , which is the time period for this gun , it was a new fangled idea which needed some time to catch on. Single Muzzle loading guns were usually carried and loaded by the shooter . By the 1850's the cartridge guns were making an appearance and it wasn't until the 1880s that the big shoots really got going and pairs arrived on the scene . I wonder if the reason it has had little use is that it was quickly out dated .
By the way I have owned rook rifles in various calibers by all the makers you mention and a few more .
Has for the pairs and trios of percussion guns these were made for driven bird shoots about the 1840,s by makers like Lancaster, Boss , and Purdy you will find that some are minus a ramrod and come in a mahogany case From the 1970,s I did in the excess of 25 years researching English gun making more so in my part of England for which I made my own directory this was mainly done because far to many authors of gun books were far from the truth so I never go by all that is written in books .