I second H&H on the Superposed 'Waterfowl' series. Saw/handled a collection of them at a DC area gunshow about a year ago and they were magnificent. I recall the asking price was just about $10K for a Black Duck 12 ga. I'm seeing these on seller sites for around $8K today.
I WILL own one of those guns someday.......outstanding.
Hammerback - look for a book entitled " Browning Sporting Arms of Distinction" by Matt Eastman. Should be able to find a copy of it.
You can attempt to find the book " The Browning Superposed "
By Ned Schwing. It will cost you though, that book has skyrocketed in price as of late.
These two books will tell you everything you ever wanted to know about the superposed shotgun.
H&H