Help removing rear lens element on an Rolleiflex SL66 50mm lens?

Trying to clean a little fungus. Looks like the rear element should just screw off, it doesn't! Tried a piece of leather and pliers even. There is not a lock screw that I can see. There is a collar around the lens held on by three set screws, but that has nothing to do with removing the lens element(had it off for a look). Any ideas? Thanks, John.

Suppose you manage to remove that lens element, how are you going to assemble the right way and align it without a collimator?

But, you said it is a "little fungus", perhaps putting the lens in UV light (sun) for a while might kill the little beasts?
It's worth the try as fungus can't stand UV...

Anyway, I would suggest to send the lens for professional repair, a Carl Zeiss is always worth some decent care...

Thee is also a possibility that the part you are trying to remove is fitted at the factory using some form of thread lock compound so after setting it up on a collimator it cannot move. Most thread lock compounds that I know of respond/soften with some degree of heat but I would not dare risk it with a lens.
Also a little trick that some lens manufacturers employ is having a multi-start thread it gives a better grip but get it wrong when reassembling the lens and you are in trouble. Leave it to the experts. It will be cheaper in the long run

In the Rollei repair manual for the SL66, the 50mm is listed as needing to be sent to factory. Some other lenses have exploded diagrams in the manual and some disassmbly and assembly instructions. But not this lens. Others here may be right- assembly requires a level of optical equipment or such that most fo us won't have, maybe factory-specific jigs.