Hi Richard.
Been traveling the last couple weeks.
Before I forget, I want to congratulate you on this barrel still having it's original pan cover.
They are usually missing. Assuming the pans that were made with a cover. As we know, some were not.
Date: Well, my guess would not be any better than your's. LOL. These guns are so difficult to try and date. The "general" overall design of the stocks and barrels seem to remain the same for some 300+ years. The gun could have been made in 1700 or 1800 LOL.
I don't even know which style/changes, etc. would be considered early versus later.
As you mention, a book could probably be written about the history/design, etc.about these guns. But the book would be seriously expensive since you and I might be the only customers. LOL
In the book Indian and Oriental Armour there is a few pages that do give some interesting details of how the barrels were made. I had to re-read it 3-4 times to get an understanding. LOL. But no discussion of interior bore/breech design. And, as you mentioned, without a removeable breech plug, it makes study of the breech design that much more difficult.
I have noticed only two variations of breech plug design. One, as used on my barrel, is an iron plug, smaller diameter than the breech, forge welded in place, with the rear sight mounted directly on top of the breech. The other style, utilizes a flat plate, slightly larger diameter than the breech, with the rear sight cut into the top of the plate. And I assume also forge welded in place.
Sometime in June, my barrelsmith will be working on my barrel. In the process, he will be drilling out the hole I already made larger till it meets the barrel walls. Hopefully, this will allow for some better photos of the breech end, and maybe the restrcted area, before installing a new threaded breech plug (and liner if necessary). So maybe we will get some more information on breech design.
Again, thanks for your discussion on this subject.
Rick