IIIas have the larger 26 spline (same as big IV, V, VI) shaft.
IIIs have the smaller 19 spline.
Gimbals were reinforced, bell housing strengthened, larger tilt pin in the IIIas.
I always thought IVs looked more like Vs. Early IVs had small shaft (19spline). All IVs had smaller diameter tilt pin holes.
Originally Posted by thisistank