Does the standard procedure apply to YA SS wheels as well, were they masked/painted at the assembly plants, reason I ask is I have never see an original paint YA with anything other than gray on the reverse.
I think you're seeing Service wheels, which were painted grey so they wouldn't rust in depot storage. Wheels were jammed by the thousands herringbone-style in rail cars, and if they were final-painted at Kelsey-Hayes, they'd have been badly nicked, chipped, and scratched before they ever got to the tire mount/balance equipment. You had to be there to understand what a rail car full of wheels looked like when they opened it.
I did a little research and The NORWOOD built 69 Trans Ams survivors which have been unearthed have grey backed Ralley II's.