Clairex was the manufacturer of analog optoisolators usually based around LEDs "shining" on LDRs. Clairex was sold, I believe, to Perkin-Elmer. They really had a fascinating product line according to the old databook I had. Remember that you don't have to rely upon a commercial product for isolation. You can roll your own and before the advent of actual optoisolators, manufacturers did just that. There were some electronic organ manufacturers that used an incandescent lamp lighting an LDR (shielded in a light-tight tube, of course), not for isolation, but as the "swell pedal" volume control. The swell pedal turned a potentiometer which in turn varied the lamp brightness. By using the lamp/LDR combo, noisy pots did not transfer their noise into the amplifier. Pretty cute, I thought.
Dean