Note
HDP-444 (Shure model 444)
The HDP-444 was released after Heath had dropped their major transceiver products and was not associated with any specific Heath rig. The HDP-444 was described as a controlled magnetic microphone and featured dual selectable output impedance with a frequency response “tailored for intelligibility.”
Photos, general information and specifications from "Heathkit: A Guide to the Amateur Radio Products," by Chuck Penson, WA7ZZE. Used with permission.