That is a nice looking Sporter and a valuable memory of your grandfather.
Jim and Scorch are right. The NRA Sporters were built at the Springfield National Armory for sale to NRA members.
The NRA did not possess, much less build or modify, any of the various rifles that came up for sale to its members from the Government.
Sales were handled through the Department of the Army by the military Director of Civilian Marksmanship.
Can't recall seeing a 1903 with a threaded front sight screw rather than just being pinned in place.
That said, it will not hurt value like the ones that you see that have been drilled and tapped for scope use.
I could see that rifle easily going north of 2500 and, depending on the percentage of finish, above 3000.
It would be a centerpiece for many 03 collectors.