I believe this type of dual gauge is mentioned in Swing's book also. I personnally inspected a 20 gauge with the right barrel chambered and so marked for 2&3/4" and the left chambered and marked for 3". Should have purchased it!
BTW Bill wants to sell his entire collection as one entity. Kind of a strange person as I tried to communicate with him once.
I have one 20 gauge out of the 'Custom Shop' and that's probably all i will have! -Dick