Ok, this part has the pocket to store the dark slide. Tried with that, no luck.
Than, and I am ashamed I did not do it sooner, I thought: Maybe it attaches to that plastic cover just like it does to the back of the camera. And that was the solution. Just twisted the Lock/Open key and could easily remove the plastic protector. I am glad I did not forced it, it would have damaged not only the cover, but possibly the entire locking mechanism.
Thank you mwdake, I found it out myself in the meantime, but it works just like you described it.