53-55 nailheads used steel camshafts and distributors with compatible gears. 56 used cast camshafts with a different compatible distributor gear. 57 and later nailheads with a different distributor can supply the compatible gear or function as a swap. Consequences of this incompatibility? "Theory" say the gear on the cam will be destroyed, but most early nails that have been rebuilt have this incompatibility and are running fine (?).