Fax STF for the Macintosh external modem. Very Rare

This is external modem labeled "Fax STF for the Macintosh," which was used to connect to a Macintosh computer to send and receive faxes. It likely connects via a serial port, as indicated by the connector in the image. This external modem would work alongside the Fax STF software to enable Macintosh users to send documents directly from their computer as faxes, and it would receive incoming faxes to be displayed or printed via the Mac.