When I had it here Omnivourious Bob stopped by to look it over with me. He said that the straight rifling was to allow the black powder crud to build up the the grooves w/o slowing down the shooting for cleaning.
That's the first one like that I'd ever seen. I still don't know what to call that hindge arraingement? Possible crab claw?

edit: The lack of any English proof marks was another concern. The only markings I remember seeing was "Smith, London"