This is one used but removed working HP Analog FAX accessory 300 aka Q3701-60020. This has the attached MT5634SMI FAX module.
This works in a large number of models of HP printers as the FAX transmitter/receiver module. Pop it in, plug in the FAX cable, set up the FAX options, and you can send and receive FAX messages using your HP Multi-Function Printer (MFP).