The back has the small button/lever that dnjl mentions and i can press it in (with a ball point pen for example). But the slide can be removed without touching this button.
The slide itself has no notches. His only cutout is the triangle which indicates how far you can remove the slide for firing the shutter without getting light to the film.
Can it be my slide is from a non-S back? (It came in a Pro-S back.)