It seems like this is what we used for music on hold.

https://www.telephonecollectors.inf...-receiver-for-music-on-hold-service/file

All of our customers were Federal Goverment entities so we didn't use them often.

Here is the diagram for the 400D Line Card

http://www.kadiak.org/tel/400d.jpg

Could be we used 4200a Line cards:

http://www.kadiak.org/tel/4200a.jpg

Sorry, but were talking 30 years ago...


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