yo soy el warg

I've got a 1988 IROC Z Convertible and have been looking up the RPO Codes and found that it has RPO Z08. This is listed as a model conversion, convertible when I looked it up. I've also got a 1989 IROC Z Convertible, but the same RPO Code Z08 isn't on it's SPID Codes. Does anybody know why? Since it seems to be the same conversion and same company that GM used to do the work. Both cars although have a "3" in the vin number for type of model which designates a factory convertible. Help me solve this mystery.