All 68 Road Runners used the same grill and paint pattern. The standard Belvedere and Satellite used the same grill as the Road Runner, just detailed differently. The standard Belvedere grill was the bright dip finish. The Road Runner should have the dark argent finish on the inner rectangle areas and the connecting pieces as I recall were a satin black with the upper and lower band being a bright finish. The Sport Satellite and GTX shared the same grill, just different emblem as mentioned above.
I have two NOS 68 Road Runner grills and darkness of the argent and the texture of the two are slightly different. I am sure that even on the assembly line, you would see slight differences between any two grills on the texture and darkness. Also on the original grills, they used a mask to spray the different colors as you will not see a distinct paint line between the argent and black.
The Decor Package and the change on January 1st to use the trunk finish panel does not impact the grill on the Road Runners.