What are these?

Hi,
today I parted out a 1980 Hammond Colonnade solid state organ. On two of the many boards I found five components that I've never seen before: 1 pc of what is shown in the 1st attachment, 4 pc of those in the 2nd one. My questions about them are:
- Am I right that VACTROL means a LED/LDR combo, althought there's clearly written »cell« on them, which might read photocell?
- If my assumption is true, the function of the four leaded component from the 1st pic becomes clear. But why do the other four components have three leads on the receiving side? How can I determine what these really are?
There's also written something on their flip side which most probably indicates year and week of manufacturing, nothing else.
Best regards!
 

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These will vary a lot in their characteristics, so watch out when designing with them,
or when replacing a bad one. The synthesizer people have used tons of these type of devices.
Some use them as a DC voltage operated volume control, or for muting.
 
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