That really threw me when i read that. I had 89' and 90'IROC's and am very familiar with all things involving them.
read about Exterior Badges. I also read a 1988 Road and Track article. The pictures clearly showed "IROC-Z" badge on the ground effects. I looked at some more pictures online of 1988 IROC's and they all have IROC-Z badges on the exterior. No Z28. Not sure how to explain yours.