Vintage Synth Repair

Can anyone tell me what the electrical component that looks like a
christmas light is in this picture? And what function it performs
(generically obviously)?

http://www.hallofgiants.com/images/misc/DSC_9087.jpg

It is from a Conn console organ (a friend's). He says they glow
orangey when a note is sounded, and I believe maybe there is one per
key. I was guessing they were some sort of opto-resistor trigger type
thing, but seeing them, I am not so sure I think that anymore, as they
are not sealed or anything. I feel like I have seen these in something
else too (a Farfisa maybe?).

Just curious mostly.

-brendan

P.S. does anyone out there have access to Conn Organ schematics by
chance (I can get the model number)?