The 28 is the Magic shot shell theory, not unlike the single bullet theory of my youth.

Another maker you missed was Crescent Arms type guns made in the US under a hundred different names. Barrels were sourced from Belgium just like Smith and just about everyone else. Another one I like was Union Arms doubles with side plates and a Lefever type ball and socket hinge joint. They only made a small number of them and I foolishly passed on a decent shape 20 bore about five years ago. Might spend the next decade waiting for the next. Only other American maker I've seen use the ball and socket joint like Lefever.