I believe that the triple pin guns were made for Egyptian poilice and prison guards so that they would not "grow legs" because the special ammo was not available except through the authorities. I remember reading an article about them in the dim and distant past, but cannot remember where, or if I remembered the details correctly.
My personal theory about .410 bore vs. 36 ga is that the Brits called it that way just to piss off the French and Belgians.