Check out the '69 Camaro Assembly Manual, Section J52 page A1. The front hub, steering knuckle and other parts were received as an assembly in production.
Standard J52 cars received assemblies #3954867 & 3954868. "RPO Z28" received 3954869 & 3954870. In my experience, std. parts had green stripes, Z/28 had pink, like connecting rods. May indicate Z/28 components had additional processing, such as shot-peening or crack checking.
Just a theory. Never saw markings on rotors.