I think the number of definitively American boxes carved out in this manner (clear to buttplate, regardless of round vs. square bottom) are surely less than 10. Deschler's rifle is cut like this, Cowan's short rifle of course, the cherry parts rifle in RCA2, #42. I think I recall maybe one or two others but it's certainly much more commonly seen in surviving German rifles. "Surviving" being the key word there. Possibly, it's an earlier characteristic and the rifles it was used upon here just simply have not survived in any quantity.
I could easily see this rifle as being either American or German - it's really a toss up although if the black walnut stock were verified it would much more likely clinch it for me. My *guess* is that when George made note of it being black walnut in RCA1, that's what he was told by the owner and so that's what he noted. Maybe some more detailed information as to the testing might be available from a previous owner.
FWIW, I think it's a really nice early little piece, although by 'early' I would personally place it perhaps in the 1760s or early 1770s. I really don't see it being earlier and I don't think that lock really predates the 1760s. For some reason I can't quite put my finger upon, I wonder if the lock may possibly be French or Belgian. Certainly enough import locks and barrels were very easily available here from merchants in larger towns and cities from all over, prior to the War. If it's American, I suspect the lock and barrel were purchased and the stocker made up the brassware needed.
The reason I think it interesting to potentially attempt to research 1864 owner is that the rifle appears to be in extremely fine condition, which may indicate very light or little use. I say this with the caveat that I know nothing of any possible restoration work. It's been my experience that pieces in such condition are potentially more likely (of course not "always," just more likely) to have passed down in families - William Marshall's Oerter rifle is a good example of this and there are others. Maybe it's just a wild thought but since it's known who had the rifle in 1864, possibly a "mere" hundred years after it's birth, it's possible that by working back through his genealogy a bit might put you closer to a maker or original owner/location. Just a bit of a curious diversion - on rare occasion it pays off.