Charles , i mean the veiwfinder (albada viewfinder) on the top of the camera that pops up when the button is pushed to unfold the camera.
it is made up of two 'lens' pieces, the front piece in the front frame is concave at the rear and its surface silver mirrored with a % of reflectivly. because it has a curved rear surface it cant be replaced with new flat mirror. i havnt heard of anyone having them re-silvered yet.
its mirrored so it picks up the reflection of the rear veiwfinder element, which is convex on the front and has frame lines for 6x9 and 6x4.5 painted (or some such thing) on the rear of it. i think from memory it is made of plastic, and this is the reason they discolour (yellow).
these viewfindas are somewhat of a problem to restore, it is a two part problem with sometimes the lens in the front frame has lost its silvering for the mirror and the rear lens part in the rear frame turns yellow, sometimes to the point where its too dark to look through. i havnt tried grinding/polishing a new surface on mine to see if that works (i vaugly have heard of others trying with no success though, its possible the yellow colour extends all through the plastic lens) as uniquely mine is quite good. I once considered making or having made some new rear lens parts and then printing the frame lines on it but necessity being the mother of invention, i havnt needed too because mine is ok.
i dont think i have come across anyone that has fixed them fully as yet...