If the numbers had been on the stock for a very long time, they'd possibly fill with dirt and grease. When the stock is sanded carefully, either the stock is already "painted" by the grease, or it still remains inside the now very flat depressed wood, being now completely flat in this area. And your stock clearly shows how this looks afterwards.

When you replace your stock with a correct stock, you might repair the currently mounted stock by cutting it below the rear band and professionally adding a new foreend. Someone with a stockless NM rifle would definately appreciate this.