Geoff:
Thanks for the additional information. Every little bit helps in better understanding this equipment.
I had considered the speaker being used to 'mic' back information, as electronically there is little difference between mic and headphone element design/function, but the rather robust look of these speakers made me hesitate about that possibility.
I dug up a second speaker and it has been repainted so much all traces of original stencils are long gone and just the 'PRESS TO SIGNAL" new printing is visible. I have a second pair tucked away somewhere and must track them down to see what they can tell me.
Is the Control Unit a rare bird? With the large numbers of speakers that used to be lying about in these parts, I am puzzled I have never spotted the Control Units or manuals.
David
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