It looks like sapwood to me too. If you look at the photo of the right side the light wood covers about 1/2 way between the nose of the comb to the heel but on the left side it is only about 1/3 of the way.

Also notice that the written description says "semi-automatic sporting shotgun" - I assume that it uses the "arm actuated" semi-automatic operating system as opposed to gas, long or short-recoil or inertia.