Well, I ended up with this gun. The original poster asked some questions about it as it was listed for sale. I have it now. Without disparaging anyone but myself, this gun is pretty out there and the breech to tang fit is the smallest problem.
First and most obvious when I got it via USPS is that it broke in half in transit. Without getting into it deeply here (it's not germane to gun building) I kept the rifle and came an amicable resolution with the seller.
I bought the rifle not for originality, but because I wanted a shooter and also to learn about the architecture and construction of this style of rifle.
After I settled the cracked stock issue from shipping I disassembled the rifle. I learned a lot. Specifically that I will not likely buy a rifle via photograph in the future. So.... What I assumed might be a bored and sleeved original barrel turned out to be so much more confounding to me. It has a liner, but it is also split/joined at the wedding band where it transitions from octagon/16 side to round. Like the original barrel was cut in half, bored then reassembled over a liner? And the rear sight is drifted over hard right. And the barrel isn't straight, Like crooked at the joint. Like someone cut the barrel in half, bored the front and back half, but not very precisely then slid them over a liner. They might line up if the boring was on center, but it's not. The front half of the barrel is clearly off to one side.
I can only imagine what will may hide behind the breechplug. The sight is hard over to compensate for the completely wonky barrel.
I still like the look of the rifle. I may inspect the breech, proof test it and shoot it. Maybe I'll just look at it. Maybe I'll re-barrel it.
Keith.