It is not anything to do with a Remington repair. Or else it would have a repair code on the barrel flats. There is thoughts that the ad in various places on Parker guns has Meriden origins and is an inspectors mark of some sort. But no one knows for sure.

Pretty sure my 16 D has the same mark. I'll check when I get back home after the 4th.

Thousands of Parkers have that same X on the forend lug. It is a maker's mark or inspector's mark.

There are also + stamps on barrel flats and elsewhere.... not to be confused with the X stamp.... but we don't really know the meaning of either for certain.