What you are in need of is the tool at the rightmost in this picture.
https://www.ebay-kleinanzeigen.de/s-...34685-230-4715
I had to deal with the same type of lockring on a 2nd gen. Cosmic. Personally I would never pay that much money for a tool I will use once. Also, given that that design dates back to the late 90s, there is a good chance the nut underwent galvanic welding with the freehub's outer bearing race. At least this is what I think happened to mine.
The way I see it your best chances are finding a store that has been a Mavic service centre for long enough to maybe still have that tool laying around, or, do what I did and drill it to destruction (it doesn't really do much).