The problem is that I can't find a way to hold the inner tub...
I really don't understand which part holds the lens because the part on the back is much smaller than a M39 thread...

I also tried to unscrew the three small screws but it seems to only hold a small plastic inner plate against the lens.

Is there any exploded view of this available somewhere ?

Here are two other pics, I did my best because I have no macro lens...
Many many thanks for all your help and any additional hints