Figured it out. Seems like 72 to 75 they retained the front cover like the round back. The stator shows between the cases. The rotor was probably same dimension. In 76 they changed covers so the stator inserts between the covers and you only see about 1/4 ridge of the stator. Stator and rotor changed. Rotor is longer.

Helping friend put chrome covers on existing alternator, and turns out the tuff item is the 72-75. So you need an early square back. Otherwise rotor locks up when case put together, because rotor is longer.

Power Master is modeled after the later square back.

Engineers always working to change something.