As I mentioned in a previous post, most of the walnut trees in the UK were cut down during WWII, and it seems that BSA used the excess supply of this wood to make air rifle stocks for some years after the war. I expect that a lot of the trees were still "seasoning" and not ready to use until the late 40's.
English Walnut has a very straight grain, especially when compared to walnut from the continent or from Turkey which we've since become used to seeing, so it appears very "plain" and might easily be mistaken for beech, maple, or (as you mention) walnut sapwood.