I recently acquired a Rollei 90 degree prism finder for my 3.5F. It was listed as spares or repair as the prism inside was loose, also there was what turned out to be balsam separation. Here's how I decided to fix the looseness. Remove the four tiny screws and the top housing just lifts off (I think the screws are under the leatherette covering on some models). The prism is retained by two metal strips on either side, held by longer screws.. I cut two thin strips of rubber, actually from a lens grip, and trapped them under the metal retainiing strips. After tightening the screws, the prism isn't loose anymore. A perished piece of foam inside the housing top was renewed, and the top replaced. Getting the screws back in is tricky. This is the balsam separation, it's visible in use but not too obtrusive, being near the edge. It's not clear why the prism was loose or whether there was foam or similar under the retaining strips, which were found tightened down. Perhaps just replacing the perished foam at the top might have done the trick, but my solution seems more secure. To be honest, I was a little disappointed with the prism in use. Viewing through it seems to make the image look dimmer than just using the waist level finder. But there you go.