At one time the center hole of wheels was not made to "locate" on the axle hub but instead the wheel got centered by the lugs and lug nuts. At some point Ford started making the wheels and hubs such that the hub centered the wheel and it no longer relied on the lugs to do the centering. From what I've read, the cut over year for this on Thunderbirds was 1964.
Does anyone know what the first year for this feature was for Mustangs? I've been told that a 68 mustang still centers on the lugs, not the hub.