The pipes really fall into a couple categories:
early or late (which has to do with the o-ring, early is a regular type, late is the double lip type). There are mounting differences early/late as well (more on that below).
manual or automatic (automatics have an extra nipple on them for the ATF cooler's hose)
Golf/Jetta or New Beetle (has to do with the angle of the nipple for the hose coming from the tank, the NB got several minor cooling system changes to deal with the more cramped engine bay, as well as changes to A/C and P/S plumbing).
Then, for the early type, there is a replacement updated version, that uses the newer type double lip o-ring,
but retains the slightly different mounting arrangement. And these updated ones are again for each of the different versions as relating to model and type of transmission.
Now, the early style pipes are all obsoleted. They kick to the updated versions, which are now mostly NLA. Which means, if you have one of these early cars, you have to update to one of the later cars' pipes, appropriate again for your model and transmission type. This will, however, also require a couple fasteners, as these later ones are attached to the block differently. And while the threaded holes will be in the block already, being that they have never been used, they may require some cleaning with a thread chaser before you screw anything to them.
You can also buy a cap and a clamp to make an automatic one work on a manual car (some VAG products had these in various places from the factory already).
I save all these pipes I come across, regardless, because I know they can eventually rust out. I had my '91 Jetta's finally rust out after two winter trips to Michigan.
