Back before the "Carterfone" decision https://en.wikipedia.org/wiki/Carterfone https://en.wikipedia.org/wiki/Carterfone in 1968, the phone company had a rule th

Back before the "Carterfone" decision https://en.wikipedia.org/wiki/Carterfone https://en.wikipedia.org/wiki/Carterfone in 1968, the phone company had a rule that you were not allowed to connect anything to their network of your own. You could only use equipment they provided that you leased from them. Hence, the acoustic coupler was used to connect your fax or computer to the phone network. The exception were special services private lines, since this device is circa 1965, it was probably intended for that. Before the internet, big companies would set up big private networks, typically Bisync or later for IBM SNA. Those private line networks were like a big conference line, sometimes referred as shout down circuits. That name came from the original use which, a common example would be a bunch of junk yards would each have a shout down and they would yell into it things like "we need a 59 Chevy Radiator" and then a guy at another junk yard would yell back "this is Bob and we have one" or whatever. These circuits were basically a giant conference bridge but not switched, it was permanently wired. These circuits later got used by computer companies by putting a "master " modem at the head end which would send out a message here is some dat for Terminal 123, which all of the modems would receive and then modem 123 would respond with some data, the protocols used were polselect or bisync but there were others. I am guessing, based on the date, that is what that modem would have been used for. Some of those goofy lines on RS232 ( or RS422) like DCD, RTS, DTR etc were used to control the modem and tell it when it was OK to raise it's carrier and reply, otherwise it would just listen for it to be called on.