Most Kombi boxes are the same with variations through the years.
Bay boxes are usually styled by the ribs on case, such as 3 rib, 5 ribs, 6 ribs etc.
There was considerabe crossover while some swear that 1800s in 74 had 5 ribs I have seen a number of 74 models with documented history with 3 ribs.
I have a very good 3 rib out of one of my own campers that is 3 rib new from factory.
As you can see gear ratios are very similar with final drive or diff ratio having greatest in 1800 and 2000cc models.
Diffs ( crown wheel and pinions) can be changed more cheaply than transmissions.
This is one reason why the VW transaxle in all forms was very popular
in open wheel race cars and the Hewland box that was even used in F1, F2, and F3 in early days was developed from the humble VW transmission.
Clutch and flywheels also differ but these can also be altered between models-mainly to suit starter motor pinions.
