That's news to me. I had always thought CdS cells were pretty stable, it's selenium cells that give out over time.
Does anyone have (or know where I can get) a service manual for this prism? I tried taking it apart today, and afraid I'd lose a screw or try to pull something I shouldn't, closed it back up without making any progress. I feel confident if I have a good idea of what I'm up against I can do it; I've done much more difficult things than this with a soldering iron and eyeglass screwdriver before.