Yeah, Ice, the 66 and earlier wheels , at least the big pattern ones, used the 10" caps. Somewhere around 66/67, they changed to the 9" caps.
As for 99% of the factory and aftermarket wide 15" steelies, they are set up for the common nine inch cap
Apparently, for a while, the aftermarket ones had too small nubs, and the caps weren't tight. Pretty sure that has been corrected.