CZJ 2.8/50 can't remove focus ring!

I bought it on ebay and the focusing ring was very stiff, so I disassembled it. All good except I can't get these two pieces apart! There are no screws in sight. Anyone has an idea as to how I can separate the two?

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thank you all

OK, I have zero specific knowledge, so this is just foolish speculation ...

Is it possible this isn't supposed to come apart? From what I can tell via an LCD screen it looks as though it could be a pressed-in ring with that internal thread. In that case it could be expected to be permanently connected and turn with that outer piece. As it turns, whatever part it mates with would be driven in or out (or drive it in or out).

My .05¢
(I'll go back to sleep now ... )

From your photo's I can't even tell if it is a different piece of metal. What does it look like from the other side? Does the threaded ring move in the black piece?

the thread moves on the black piece, and it's movement is limited by an inside screw (hidden). my guess is that I'm going to need a lens spanner or some similar tool to unthread the metal thread (it's threaded on a finer thread)