The stop rings that are normally rusted and hard to remove can be replaced with the ones from newer Ducs. The old stop ring has 1mm thickness, the new one 2mm so with a small grinder you can enlarge the groove. It`s an easy job and done in a few minutes.
The new stop ring is much easier to position and to remove again.
Note that as the new stop ring is thicker the groove also needs to be shifted slightly outwards as otherwise the ring won`t fit if the seals are mounted. (Be very careful doing this! Once ruined there`s no way back)
Of course you can also leave the fork as is and grind down the new stop ring to 1mm thickness.

G.

saw in a catalogue that the seals exist in different heights.
The original ones are 41,7 55 10/10,5 (inner diameter, outer diameter, height/height of seal lip)
Replacing the stop rings with the newer type would be easier when using 41,7 55 8/10,5 seals.

Btw. the last time I bought a pair from my Ducati dealer they costed about € 33 if I remember well and some bike shops here (POLO, Hein Gericke) have them listed for € 9,95 !

G.