I have a J&WTolley catalog that covers the fowl guns as well as the big bore rifles. Has the Ubique ball and shot guns as well.

Pictured in the catalog are large bore SxS fowl guns with two-leaf flip sights for long-range accuracy

I really doubt a 4-bore rifle would wear 30'' barrels
A rear sight on a 30'' 4-bore SxS fowl gun doesn't surprise me at all.

--Tinker