I found a source for these in used but excellent shape for eight bucks each.
Roanwell Hawk Series
Lightweight headset for maximum comfort and reliable performance. Worn with hard hat or helmet without fatigue. Lightweight cords allow freedom of movement. Soft padded headbands adjustable to keep headset in place. Superior voice transmission provided by noise cancelling carbon, dynamic or electret microphone. Soft foam filled or gelled pads provide comfort. Available as single sided (Type 490) or double sided (Type 491) for moderate noise areas. Both configurations are also FAA approved.
Are they worth it? What do they fit? What can they be adapted to?
LATER: The marking on the plug is AMPHENOL 67-06J14-9P. The cord running down from the left earpiece on the headset is marked: 491-049-001-694 G-1 12 coil cord.