ZF Transaxle Types

[BruceA]

Apart from the usual side ID plate, what is the difference between the 'boxes' with regards to the side plates around the diff area? Some are fastened by, I think 7 bolts and others 4 or 5? Is this an indication of strength upgrades over the years or something else? I know the original design is quite old, but is this also a representation of age??