How can the lever of a special single-lever mixer be removed?

I recently bought an apartment that I want to rent out. It is fully renovated, only in the bathroom above the bathtub the single-lever mixer does not work properly. I want to completely remove the faucet (see picture) to find out where the cause lies. However, first the lever must be removed, which is proving problematic.

Although I am a trained mechanical engineer, I have not yet succeeded in removing the lever for cold and hot water. On the underside of the lever there is a bore hole about 5 mm in diameter. At first, I assumed that there was a screw inside, which could be loosened to remove the lever. I thought the screw could be loosened with an Allen key, Phillips screwdriver, etc. But far from it, since I have not been able to see any screw head inside the lever so far.

I shone a flashlight into the hole but could not see what was inside. In any case, there is nothing screwable and nothing that can be moved at all.

Apparently, the faucet is from Geberit, manufactured around 1995.

I am sure that an experienced plumbing specialist knows exactly how to remove the lever.

I would be very happy if someone could give me a tip.

Kind regards
Harry

Have you screwed on the gear shift lever, the small one I mean? Then the large one should only be a cover.

You are right, the small lever was fastened with a small grub screw with an internal hexagon and the large one is just a cover. Now everything is fine again and works perfectly.

and the large one is just a cover.

Then take another photo without this cover and help the next person who faces the same question.