Thermal flow shocks rebuild

Hey guy's,

I ran into an issue while trying to rebuild the original rear shock absorbers of my 74 DT360A.

After removing the spring, you have to loosen a special groove nut that holds the internals. I tried to undo it with an adjustable tool for angle grinder disks, but the material of the nut seems to be quite soft while the nut sits pretty tight at the same time. Is it even a normal right handed thread?

My last hope would be a special socket for the impact wrench, but to be able to get it on, I have to remove the upper bearing eye from the main rod. Does anyone know if it's screwed on or how do I get that off? Maybe someone even has an explosion drawing or a rebuilding manual. I wasn't able to find anything on the internet, even though these shocks were used in many other Yamaha DTs.