| Jools,
Just had a look at the parts manuals for the ST2 and the ST4s. The part no's for the spindles for both bikes are the same - so no problem there.
The steel swinging arm is clamped by pinch bolts threaded into the swinging arm in front of the pivot from underneath.
The alloy swinging arm has pinch bolts behind the pivot and entering from the top. The alloy is presumably too soft to take the torque required so there is a nut on the bottom of each pinch bolt, hence the holes.
My bike is at home in the garage so I can't pop out to have a look to confirm that but it is pretty obvious from the parts list. | 
Threaded Mode