I don't know how much stronger the late IIA box is compared to a SIII box, but I've many times read articles in LRO etc which go on about how superior the IIA box is to the SIII. The only breakage I've had on a SIII gearbox was an input shaft bearing, but that bearing is identical to the IIA one. A significant difference between the two gearboxes is the layshaft. They have the same size bearings but the SIII layshaft is a one piece design whereas the IIA layshaft is splined and all the gear wheels slide onto it. I would imagine that the the latter design is stronger because the grain of the metal in the shaft and the gearwheels can be ideal and there would be less stress risers. I would guess that the grain of the metal in the SIII shaft are a compromise and that the design was introduced as a cost cutting measure. I think a SIII layshaft is very expensive because of it's complexity. If you strip the teeth off the first gear on the layshaft on a IIA (I've seen that) then you can just slide a new one on for probably less than £10.