True regarding barrel keys, and I believe the military pattern New Land Light Infantry would also have a scroll trigger guard and a rear sight.
Although I am not a student of the Brown Bess and have much to learn, a variety of models have passed through my hands over the past 50 years. If I am not mistaken, private purchase muskets could have an odd assortment of features found on military pattern muskets. I based my suggestion on the butt profile, which I had not previously seen on a Third Model Bess. I would think this piece to be a private-purchase Bess having the stock profile of the New Land Pattern Light Infantry and features, such as barrel pins and possibly bore diameter, common to earlier military patterns. As stated, I am far from an authority on the subject and could well be mistaken. What follows is an honest question that is not intended to be argumentative.
Is there documented evidence of the military pattern Third Model Bess having this later style butt and when would it have been introduced?