I have seen various shades of walnut, possibly different walnuts from differing countries/climates.
The three piece jobbies could even have different colour walnuts composing the butt and the forends - which also looks wierd - well to me anyway!
BSA did have problems on occasions obtaining walnuts for their stocks - I have heard/read that before.
The guys that would know all this are most likely long gone now - Bless them!

ASM