The manual of the RB67 Pro-S says that the darkslide can't be removed from the Pro-S back when unmounted from the camera.
I have two Pro-S backs and unfortunately only one darkslide. On both backs the darkslide can be removed without problems. Could it be I have the wrong darkslide? Should there be some cutout in it or something?

Also, what is the little lever marked with an arrow underneath the wind lever? (I don't mean the multiple exposure lever.) Nothing mentioned about it in the manual. Even not numbered as other parts are.