IF I remember correctly it's the spline count on the axles.
I know a '63 Riv. & full size Buicks are 30 spline.
I believe '61&'62 have 31 splines.
One of the reasons a '61 or '62 are favorable is because they will replace the torque tube from '56-'60 as a bolt-in to make them open drive.
Tom T.