I checked my rpo's on my 1990 IROC and one of the numbers is "Z28" I found an internet site that said if you had an IROC that the RPO was something else. Since there were no Z28's in 1990 I am lead to believe that this code is for the IROC package, am I right?

Yup. Through 87, the IROC package was an option package on top of the Z28, and it had RPO B4Z. From 88-90, the IROC option consisted of everything previously available between the Z28 and B4Z option combo, so they just kept the Z28 RPO code.

oops

Yup.

85-87 IROC-Z: B4Z and Z28
88-90 IROC-Z: Z28