That's a GREAT Superposed!!!
I inherited my dad's 20ga Superposed and let my grandson use it for his first bird shot on the wing eight years ago. I was with my dad at The Sportsman in Reno NV in 1957 when he bought it new for $235.00. It's a bit unusual, as it has a solid matted rib. It's not nearly as clean as yours and shows decades of use.
You have something very special.