When comparing the BBL signature on the subject rifle to those on the known examples, one can see a significant difference. While the signatures on the known examples have a pronounced lean, the signature on the subject rifle is pretty much vertical. It is particularly noticeable in the B, h and l. Although Wm's signature might have become less refined in later years, I would not expect to see such a change in his writing style and am led to conclude it is likely a poorly executed forgery.
Having only had the privilege of handling one Buchele rifle, I am totally unqualified to comment further. My strongest memory of that rifle is how incredibly slender and dainty it felt.