It think I'd worn my Zenit E out by 1972 and replaced it with a Prakticamat
My first thoughts from larger Reflex camera roller blind shutter restoration/repairs is that it's the screw on the right that's the key, that locks the other in place and then the one marked is turned to adjust the tension, that's why the outside is like a gear (for lack of a better term) so it can be locked in different positions.
If that wasn't the case the screw on the right would be redundant
See here the right hand screw is removed first.
Ian