The factory GM ones are actually made from machined brass. I have seen cast iron ones. Those are aftermarket ones as well. Though they make machined brass aftermarket ones. Brass aftermarket is closer to OEM in construction.
No the factory G body are cast iron. Trust me I have lived GM all of my life before coming over to Mopar because dad was Mopar. The replacement that is sold for them is Brass. I worked at the dealer when they were new also.