Why would a London shop, with London craftsmen spend time on the common A&D gun when it was B'ham's only game in town.
What about the Best Original Quality grade SLE's that came from Birmingham? Makers in Birmingham certainly made a lot of plainish A&D's, but they also made a lot of fancy ones. Oh, and all those provencial SLE's and A&D's; what of them?
London, Birmingham, and the provences were all members of a trade that competed fiercely and cooperated nicely. "Work" was judged on quality required, time allowed, and cost incurred. "Work" went to shops on an economical basis. Remember, most shops were run by master gunmakers and any master could get a "best" gun out the door - the much bigger problem was getting a commission in the door.