Nice detective work on the differences.
If you go to buy parts from one of the on-line stores, the later ones are called 'revised' squarebacks.
Heads Up, The Tuff Stuff Alternator I bought is a revised squareback.

It seems that the revised squareback rotors can draw more current when full fielded. We can check that with the service manual specs. I have suspected that this load is more than some regulators can handle. Probably more of an issue when using with an transistorized positive controlling regulator like VR128 that was never made for those currents.