There is a yenko.net member with a 68 RS/Z that was unrestored and did have the original grille in 1990.
Lives in CT... I have pics of the car before it was restored and it had the horizontal silver bars in the grille.
I just sent him an email with a link to the thread.
I own the car that James is referring to and would be happy to help if I can.
First the bad news. When I purchased this car in 2001 the original grille had been repainted all black and there was a damaged section so I replaced it with GM parts with silver applied to the horizonatal bars. I gave the grille and trim pieces to a friend for his car and that car was subsequently sold so I do not have that grille to refer to.
Now to the good news. I have a photo of the car from 1990 showing the original grille with the silver horizonatal bars. I can post that if that would help. I also did a fair amount of research to recreate as close to possible the rivets that fasten the upper and lower trim pieces to the center grille and also the rivets that fasten the trim pieces to the headlight doors. I believe I have some of these fasteners left over and if I can find them I can post photos, if anyone is interested and if my cheap camera will work.
Also if it is helpful I can dig up photos taken in 1968 of a family owned 68 RS SS 396/375 car showing the front grille.
The Z is a Norwood built (July 68) car and the 396/375 car is a Norwood built (Oct/ Nov 67) car.