This may answer your queries. But be careful as there were other boxes, depending on what part of the world you are in. For example, there were military versions here in Oz that were very strong.
" The gearbox used in series Land Rovers was first used by Rover in 1932. This gearbox has undergone several modifications since then but is still basically the original design. This gearbox was considered advanced and strong when it was introduced and was finally discontinued because it could no longer be modified to hold up to the power and weight of newer Land Rovers.
From 1948 (first LR production) to 1950 the gearbox had permanent 4WD with a freewheel device in the front prop shaft. From 1951 Rover fitted a transfer case with the now well known 'yellow and red knobs' right up through series III.
The dipstick and top oil fill disappeared with the introduction of the "B" suffix gearbox. At that time the transfer case intermediate gear shaft diameter was increased for additional strength. In the autumn of 1963 the gearbox ratios and transfer case low gear ratio was changed with the introduction of the "C" suffix (See my gear ratio web page for actual ratio numbers of the different series and Coiler gearbox & transfer cases).
In the Autumn of 1966 Rover introduced the '1 ton' 109" with the 6 cylinder petrol engine (A small number of 4 cyl models were made in 1970/71) . The 1 ton version had a lower ratio gearbox and transfer case to accommodate carrying a heavier load, and to compensate for the 9.00x16 tyres that came stock under the 1 ton model. One ton models also had ENV axles instead of the standard rover type. The 109 forward control Land Rovers used the same gearbox and transfer case.
The factory built a small number of all syncro series IIA gearboxes just prior to converting the box into the series III gearbox. The all syncro series IIA gearboxes can be identified by a "S" prefix on the stamped number located on the transmission top cover. Since top covers can move about when multiple gearboxes are rebuilt, and a IIA gearbox is most likely to be rebuilt in the earlier documented style, the 'S' stamp on the top cover does not guarantee the gear box under the top cover is still all syncro.
The Series III gearbox has syncro in all gears as well as lower first and reverse gear ratios. The transfer case remained the same as was used on the late IIA. Series IIA gearboxes (D suffix & later) are generally considered to be stronger and longer lasting than early series III gearboxes (The ones imported into North America). Though a lot of this conception may have to do with people's shifting habits.
There was a final change made to the gearbox about half way through the series III production (After Rover left North America). These late series III gearboxes are considered by some to be the strongest of the Series Land Rover gear boxes.
In general the Series gearbox is considered to be strong enough for about 120 HP, 160lbft is about the limit on torque. Early gearboxes are considered to be weaker than later IIA gearboxes. Early gearboxes tend to beak layshafts and mainshafts at the circlip groove positions. Late 2A layshafts without the circlip groove are stronger than the early ones. According to Bob Shannon "Here is the fix for the earlier gearboxes. The layshaft from suffix D onwards gearboxes (part # 556040) has a stepped shoulder that fits into the rear of the D-onwards series layshaft 2nd gear. So you don't have to buy a matching set of gears (D-onwards is part # 600916) you can take an earlier layshaft gear from the pair (Suffix A-C is part # 245766) and machine out the relieved section to replicate the later gear's greater clearance. Also a suffix D layshaft is probably cheaper than the suffixes A (part # 09917) and B-C (part #528703). Make sure the mainshaft is set up correctly!!! "
The all syncro transmission has a reputation of being less robust than the earlier versions mostly because they do not hold up to quick shifting. The syncros must come to a stop when the shifter goes through the neutral position if the syncros are to last. When shifting an all syncro gearbox it is advised to hesitate just slightly in the centre of the 'H' pattern.”
The gearbox specification was changed in 1961 with the introduction of the Series IIA. The main gearbox had a beefed up lay shaft and a front bearing as well as a repositioned reverse gear idler (these used to break sometimes). The idler pin on the gearbox was also enlarged. This gearbox could even cope with the 6 cylinder engine without problem.
In 1971 an all new gearbox was introduced with the Series III. Basic differences were that this had a larger diaphragm clutch (Note: this larger clutch plate and diaphragm will fit on the older Series IIA bell housing) and the 4 speed synchromesh. This gearbox needs to be treated sensibly. The massive baulk ring assembly does not like to be rushed and can cause problems if it is. This gearbox is not considered to be as strong as the Series IIA gearbox as people tend to be rough on them be trying to power shift. "
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