Magnifax mask mysterious knobs

I can't believe I've been using this mask for the past five years and didn't figure out what are these for. Help anyone?

Those are adjustable guides.
Typically used in their pulled-to-center position to guide 35mm to be in optical center.

To be honest, I didn't exactly understand, I am looking at it and trying to see how it works but no luck...

Longitudinal film positioning guide (adjustable for different formats.) I think that is common on adjustable-mask negative carriers.

In the position seen in your foto the guides will put 120 film in the right place for printing. If you move them to the other end of the slot, the left in your foto (ie. towards the front of the neg-carrier), then they will support 135 film in the centre of the glasses. The space around the 135 negs can be masked with the arms built-in to the neg-carrier. Presumably you have been printing 120 for the last five years Meopta made some nice gear and I am happy to use it myself.