The common Y block distributor - post '57 - is the Autolite, has the round grooved base and the internal flyweights like this photo ... slots in the older upper rotor control the advance.
The newer Motorcraft version has the triangular base described earlier - often used by rebuilders to make a more "modern" Y block distributor. That unit can be modified by replacing its point plate and upper rotor with parts from a '75 and up Duraspark unit.
