For what it is worth, I have never been aware that any British gun MUST have a serial number. I seen plenty from right up to between the wars that had no numbers on them. It usually denotes a trade made gun bought in completely finished, most probably from existing stock. Many have no name engraved on hardened metal either.
My researches of muzzle loading guns by J Blanch & Son have shown a mixed bag of some with numbers and some without. Most of the better examples had numbers but not all.
My personal feeling would be that if the style and quality is right, I would accept it as a Boss.
