If I were to guess, I would bet the X mark belonged to the facility where most of these were sent for refurb. M91/30 refurbs also all have a refurb mark corresponding the the Grau facility where the work was done. It would stand to reason K98k's were sent to a facility that specialized in them, as there were enough rifles to merit that. That facility's mark was likely the X we are used to seeing.
On Mosins, you see diamonds, triangles, boxes, squares, circles with an X in the middle, etc. and these all denote specific repair factories. So why not just a plain X for the captured mauser refurb factory?