You know what always make me wonder about the SIIa and SIII mounting of the pintles is the size of the bolts. The original bolts through the pintle and larger than the 4 bolts that actually hold the adapter plate onto the chassis.
The plate across the rear of the SIII actually plays no part in securing the pintle to the chassis, other than making the bolts 6mm longer.
All seems an engineering oversight to me!