those were the original Bell "modems". i remember seeing two basic models, a 150 baud (in a TI microcomputer) and a 300 baud, but there may be earlier ones that were 75 and 100 baud (there were also some that were 45, 50, 60 and 110 for actual teletype machines so they could be used without expensive leased lines)). they didn't use any quadrature modulation, just straight FSK like a teletype.