There are countless examples where different VW parts share the same 9
digit number but have a different letter suffix and are in no way
interchangeable. Don't assume that a later letter means a newer or
better part than the older letter. It often simply means it is for a
different application and won't work for the other letter, at all. Yes,
sometimes a later letter means an improved version of the same part but
often times not.

As an example, for Vanagons with 2.1 engine, look at the hoses.

Part number 025 121 058 is for a hose
It comes in letters B,C,D,E,G,J,H,M
A given van uses almost all of them, in different locations.
They are different diameters, different shapes, different lengths.
Same number, different letter, different parts

In other cases the same part number is used for different age vehicles
but with slightly different parts going by the letter. When a newer part
comes out for the older vehicle it can get a newer letter than was used
for the newer vehicle. The sequence of the letter in the alphabet tells
you nothing as far as which part fits the older or newer application.
The newest letter might fit only the oldest application.

Mark