I have a 1967 327/300. I recently discovered I had a RC-6, 13# radiator cap on the expansion tank. It has been on the tank for years and I never looked at the stamping on it because it looked "original". Now I know it is not the proper cap for my car. The RC-6 cap has a brass center. I have seen the post that suggested the RC-6 cap might have been a service replacement cap for 1958-1959 Corvette's with FI. Was the RC-6 cap used on any other Corvettes?