The seal you are referring to installs inside the axle housing at almost the end of the axle shaft, just inboard of where the hub fits on. T
om Endy's article contains a lot of information you may find useful. Also, if you do not already have a copy of
Les Andrews Red Book, do yourself a favor and get one! There is excellent information on rebuilding the differential in it, including expanded drawings.
A-4221 is the differential bearing that installs on the ring gear case, not the seal. The seal is A-4245. You can simplify your life if you use A-4246 (Snyders number) to install it. It uses pipe with 1/2" NPT threading, which may be a bit problemmatic to obtain in Germany. Check for wear on the axles where the seal rides.