The correct cap is an RC-15 Glenn. The last use of the RC 13 was in 1964 on non A/C equipped cars. You want the one with the "big" ears and are cad plated. The newer stainless ones with the small ears are not correct for our cars but work fine if you're not super critical about it being spot-on. There are also hollow and solid rivet designs. The repros are really bad, even the ones available fro the Corvette sellers. They look OK, but the spring and construction are piss-poor and they rust out very quickly and leave a rusty mess in your cooling system. You can have an original rebuilt and replated for about 225-250. NOS ones once in a great while can be gotten for 200 +/-, but will usually trade in the on the 350-up range. Good luck with your search.