Modified motors seem to have their stators and rotors in diffrent physcal positions from their stock counterparts. Using these reedy sonic rotors as a clue it appears that the mod rotors have their magnets closer to the front of the motor...
Sonic Stock Rotor 12.5 x 25.0 (Part #956)
Sonic Modified Rotor 12.5 x 25.0 (Part #958)
... because that little brass collar is spacing the rotor away from the endbell/timing assembly. What I don't get is why?