That makes sense as one of the rear brake disk conversions I found asks if the drum slides on or is pressed on to the hub. Mine is outside of the hub (drum slides off the hub after the wheel and 2 flat head screws are removed). The location of the drum relative to the hub must translate into location of the disk relative to the hub affecting the needed depth of the rotor.
However, I would think the version with the drum outside the hub (like mine) would require a deeper rotor. Maybe the deep rotor for the 62-72 needs to be even deeper for the 47-62 version?
