Walt,
Could have made it's way here via another country as a used gun. There is also the possibility it simply didn't get marked with an importers name (hey, it happened, back in the day). From what I could see, the wood appears to be very nice. Neither US importer I mentioned was known for nice wood in their offerings, but, it did happen from time to time.
I doubt the buttplate is a replacement. They are a composition of a primitive plastic, horse hooves, and lord knows what else, and tend to shrink, over time.
The barrels without under rib are common on Darne guns.
Certainly a useful gun if you can make it work.
Good Luck.
Best,
Ted
