Are you ready for this? We are talking standard transmission units here.
There were changes to series one box's during production and there were changes to series 2a ones as well.
Times the transmission remained the same were from Series one 86 models [1954] up to series 2a models until about the end of 1966.
I once had an early series 2 88 that had a series one box, only outside difference I remember was it had a dip stick on the side.
If asking for Lightweight 2a in your avatar then likely it has the 67-71 box that has different ratios.