Wow, you did you homework. But again, the center cap has nothing to do with this change in the production of the wheel. The bare wheel sitting there side by side "early" and "Late" (I think they changed it even before the 66's came out) look exactly the same EXCEPT for this one difference. They are the exact same size, the exact same stampings, just one little difference.
A hint, it had to do with metalurgy (if I am using the correct term, if I am not and I send you in the wrong direction, I am sorry).