Hi Bernd,
here is a picture of the 2.5x objective
The cylinder containing the front optics A is locked to the cone B also with the screw C removed.
By holding firmly the ring G the cone and the cylinder rotate up to about 120° relative to F when a strong rotatory force is applied to the ring D.
The ring F rotates freely with respect to te objective body H, An eccentric movement of the other side of the optics containing cylinder is detectable.
From the english reference the cone should unscrew easily, but is not my case, so I concluded that two things can happen:
1) The limited rotation is due to some hardened grease on the cone threads.
2) While rotating, the optics cilinder and some other parts bound to it moves downwards and encounters a stop.
I hope to have been clear.
Regards,
Salvatore