Hmm, good question, my friend. Later flint English fowlers, singles as well as doubles by Manton and many others look like what we call "shotguns". So I'd say from about 1790 at the early end, there would be some English fowlers that begin to look pretty familiar in lines to "shotguns". I'll go out on a limb and say that by 1820 single barrel halfstock fowlers were probably predominant in the settled east. Fullstock trade guns were always cheaper to make and sell than halfstocks. Lower cost and tradition may account for fullstock trade guns being made much later than this.