Wouldn't that be a wonderful find if it were a Haines!
At first look I thought a European gun, or a New England piece as well. But as early as it appears to be I wouldn't rule out PA as well.
As for the hardware, I have an early fowler stocked in curly maple with almost identical hardware and engraving, plus as Guy points out there's a couple others in RCA as well, so we know that this furnature was available and used on some early American made guns.
As for the walnut being able to to pinned to one side of the pond or the other, don't hold your breath on that. I once sent a wood sample from an ancient chair to the forestry Service guys to analyze, and it came back as either English or American. However it would certainly be helpful to send a sample of this gun to them on the chance they can pin it down.
If the wood can somehow be proved to be American, this gun will be an important early find, no matter who made it!
Thanks for posting the pictures!!!
John