I think you are confusing differences between gearboxes with the number of cylinders. Just to clarify a few points.
Difference between 4 and 6 cylinder gearboxes :- Bell housing only.
Difference between Series 3 and Series 2a:- Series 3 has an all synchromesh gearbox and a different clutch mechanism.
Other differences:- In both the Series 1/2/2a and Series 3 gearboxes, there were a number of changes to shaft and bearing sizes and gear ratios etc. These are particularly numerous for the earlier gearbox, which I think was actually introduced in 1932, but was essentially unchanged in the Series 1/2/2a until the Series 3 in 1972, but with numerous minor changes. These are identified by the suffix on the box number, but you can still be caught, as some parts are interchangeable in sets, and they may have already been changed from original.
John
John
JDNSW
1986 110 County 3.9 diesel
1970 2a 109 2.25 petrol
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