If my memory serves me correctly the # indicates nearly some of the last receivers made. Inside of two or three thousand maybe. Probably from left over put together parts which may explain how it wound up with the small hole N.S. bolt vs the slightly earlier NS
Could have been fitted with a bf bolt too and it was just swapped at one time but they are long and this one closes real nice on a Forster go gauge with good lug contact. Lost to the ages I guess. Butt-plate is smooth.