The only difference between the early Supa5 and later model gearboxes is the slot in the input shaft to drive the oil pump. Also the oil drip tray is exactly the same in the Supa5 and Saxon (ETZ301) engines I have worked on. I’ve also never had any of the gearbox problems described, so I reckon that’s just a phase they went through. One of my current Saxons is running with a few gears from a Supa5, but I can’t remember which gears or which engine!