IMO, it may have been a Westone with a Yamaha sticker on the horn to fulfill an endorsement contract, possibly a early version of the Super Headless. There are other period videos where Michael Benson is playing Westones, but they are a different color than the ones shown in the earlier posted Youtubes.
What it is not, is a standard Yamaha headless of the time...nor is it an Ibanez Axstar, a Cort, a Gordy, or a Hohner Jack. Interesting thread.
Bongo Club #76, Headless Club #5, Team Trace Elliot #140