On Sun, 26 Nov 2017, Ed via cctalk wrote: "What did it go to?" It connected a terminal, or computer in DTE mode, to the phone line. It looks like a fairly ordinary modem for its time. The acoustic coupler connected to the box, and the terminal/Computer-DTE connected to the DB25. It might be Bell 103 compatible, but in those days, not necessarily. The other cable looks like direct connect to phone line, without acoustic coupling. (cf. Carterfone V Western Electric 1968?) No idea what the extra plug (loopback?) was for. A few years later, copanies such as Anderson-Jacobson and Livermore Data Systems stared making more compact modems with acoustic coupler on the same box.