Following photos are step by step instructions for disassembling the rear group of Carl Zeiss Jena Flektogon 20/2.8 M42 lens. I had to do this operation since the inner surface of the rear lens group was contaminated by the aperture blades. I cleaned the surface and all became perfect.
I sold this lens some months after doing this repair, so I don’t have it anymore. I didn’t do any other repairs on this lens, therefore I won’t be able to help with your questions on other parts of the lens.
I will update the text on the images whenever I can. For the time being, please follow the set below: