61665 is used on the impulse coupled mag (Bendix, Slick)
61663 is used on non-impulse coupled mag (Bendix, Slick)
72495 is used with the Bendix retard breaker mag.
IIRC, the 61663 non-impulse gear is also the one used on the P(E)-Mag. Check with Brad to make sure.
If I have any errors, Mahon will point out my error.