Note that the breech plug is darker than the rest of the barrel. Sometimes gunsmiths dunk a barrel into their bluing tank to loosen the breech plug. I cannot see any hint of damascus pattern, but the pitting may have obscured it. Was Whitworth making liquid steel barrels in the early 19th Century?
In passing, the gun fits me perfectly. It has a very thick wrist,fat butt stock and weighs, I'd guess, around five lbs (less?). It might have a tad of case off. Handling it one can easily understand how old flinterers could have been fine shotguns.