The "A" hinge is an original 3 bolt door hinge. This covers all Notchbacks and Variant up to early 1966. These hinges are 'corner specific'-meaning that there's a unique hinge for each corner of the car.
The "B" hinge is a conversion hinge which allows the later 2 bolt door hinges to be used on the platforms listed above.
The "C" hinge is an original 2 bolt door hinge. Covers Variant/Squreback and Fastback from 66 until the end of production.
The "D" hinge is a conversion hinge that allows the use of an early door on a later car. This hinge would be used on an early 66 and earlier Variant door or a Notchback door on a 66 and later Variant/Squareback or Fastback. Like the "A" hinge, these are 'corner specific' with four different part numbers (one for each corner).
It's a little more complex than it needs to be because Volkswagen often updated/changed parts mid-production vs. waiting for a new production year. Very common on 66 and earlier cars actually.
I'm left to wonder why they didn't just keep the door hinge design the same and eliminate 50% of these designs.
The full story can be found here: http://www.neil.us/type3/doors/
A quick search in the classifieds turns up quite a few of the "D" hinges.