Hi Simon
The SIIa suffix D is actually the stronger boxes because of the change in the layshaft, where the circlip, retaining 2nd gear was removed and replaced with a shouldered layshaft.
The suffix B and later transfer boxes are more reliable than the Suffix a and SI/SII boxes because of the larger intermediate shaft diameter. The best low ratio crawler ratios are obtained by using a suffix c+ gearbox with a suffix B transfer ratio.
Also on the late SIII boxes the casing were reinforced with extra alloy (lattice shape) over the reverse idler shaft. The early SIII boxes were renown to crack in this location.
Can be seen on image below between red and green arrows.
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