I have been in the hobby 41 years, virtually all of it centered on the ’69 Camaro. The “black Z/28 grille” yarn has been out there since the cars were new. If you flip through late ‘70s car mags it won’t take long to find a non-RS ’69 Z/28 pictured with a black grille. At the time many believed they were built that way. Many people also believed you could order one with headers and a cross-ram. It took years to dispel these myths; in many ways Jerry’s book was the catalyst. His books remain the best source of Camaro info available and have stood the test of time.
When the CRG was formed 18 years ago we had to deal with anomalies in production. Thus the phrase “normative practice” was developed.
http://www.camaros.org/geninfo.shtml#Normative
I suggest you carefully read the last paragraph. Just because the car has a black grille does not mean it was built with it. Talking to previous owners is a good place to start but oral history really isn’t proof. The car could have been damaged in transit or shortly thereafter and repaired with a black grille. 47 years is a long time to remember an insignificant detail and it is entirely possible even the original owner was not aware of it. It’s also possible he painted it.
If it was in fact built that way it was an error. Standard practice was a silver grille; about 16,000 non-RS Z/28s were built with one.
Good luck in your research.