They had some differentiating characteristics, if I remember an earlier thread correctly. I think for one thing, the rims were blue. I can't remember if they all had rallys like the one in Charlie's pic, or if some had dog dish. But in any case, the rims would not have been blue. I think they were special ordered by NASCAR, and picked up at a dealer, somewhere near the track. I talked to a guy a few years ago who had one from Tallagega, I think. He thought it might have come from a dealer in Chattanooga.
When I was looking for a car, a guy told me about a 69 IPC under a tarp in a carport. I tracked it down, and talked to the owner, who also thought it was a IPC....but it wasn't Z11 nor hugger houndstooth. Other than being a white SS convertible, there were no other similarities to an IPC.
After the discussion about the NASCAR pacecars, I drove past the car again, thinking it MIGHT have been a NASCAR car. I didn't remember, until I looked again, that it wasn't even an RS. Now if some of the NASCAR cars were not RS, I'll try again. Anyone know?