Odds are the whole butt was severely rust pitted to the point of needing to be restamped once it was milled somewhat smooth again. (Thus the need for the serial number to be restamped on the butt.)

As far as the serial number goes.........if the serial number restamped on the butt is the exact same serial number stamped on the rear facing flat on the yoke it could be reasonable argued it is the original frame serial number but only if the frame and yoke also have matching assembly numbers.