Warning: from Olexander - see description (click on photo)

Olexander - "Ater this operation, you can destroy of previous adjust of position of ground glass inside videwfinder and you can see of parts of ground glass only.
From practical side, the position of of ground glass inside viewfinder depend from volume of screw tightening."
The dust on mirrors not influence on quality of image on viewfinder too much, because, all this dust outside of focus.