Disassembly of pre-war FED f:6.3/100mm?

[Alex Kraus]

Does anybody in this forum have a clue on how to disassemble the 6.3/100mm or Macro Version of the 3.5/50mm?
Both have very similar barrels. Specifically, I would like to know how I can remove the aperture ring in order relubricate it. And I'd also like to chemically blacken the aperture blades after removing the front and rear elements.
Problem: There is no screw that can be taken out in order to release the aperture ring. My guess was that it is necessary to access it from the rear side. There are 2 flange rings around the rear element. The inner flange ring unscrews the rear optical group itself and gives access to the backside of the aperture. The outer one can be also unscrewed. I did that and expected that it would allow me to pull the entire optical front group out of the barrel.
But it didn't do anything! This flange ring (it is actually a 1cm tube with inside thread) doesn't seem to have any function. Everything remained thightly fixed and I couldn't see any option to proceed further. I did reassemble everything back to its original state and now I'm hoping that someone could give some advice