try taking the photo in a dimmer room without a flash in "Night Mode" you may be able to distinguish the serial, or go to a Military Surplus store get what is called sight black spray some on the area where the serial number is, use a small block of wood and an old t shirt, pull the t shirt tightly around the block and rub across the sprayed area, if any concave areas exists the sight black will stay within the crevices, revealing what is left of the serial number. Or maybe there was an error in the stamping of that particular rifle and it was never stamped, If in fact it was sanded off there would be alot, and I do mean alot of metal missing. making it an oval or egg shape. im sorry I cant help more. hope it works.

To see if its been sanded, take the bolt out and look down the rifle at a sharper angle, if there is any metal missing, you will be able to tell. it should be perfectly round. anything less means its been tampered with.