Sir Charles,
I, in all honesty do not know.
There are a number of really fine grain very hard woods that could serve this need.
I have to admit that I was unaware of American Holly, in clear grain, large enough pieces for butt stocks.
I thank Trooper Hook for his posting in this matter.
American Haughthorn (spelling) = "American Iron Wood". But this has only been in the smaller sizes like you may find for knife handles.
My best,
Blair