The rifling as you describe in the Rt bbl suggests W.W.G. and is reminiscent of a B & S gun I had, though mine had both tubes rifled. It was signed A. Hollis, but a gun man of note who inspected it was certain it was built by Greener.
Looks like yours was made for someone who valued a tight pattern AND the ability to use ball.
No rear sight(s)?
Love to know the choke in Lt side.

Pete