Jan 10, 2018
Not first hand knowledge, but going by an article in Melody Maker (Aug 1970), he used an AKG D109 'Throat' mic. I think he made the attachment clip himself.
Just recently, video footage has been made available (Roger Trowbridge - 'Playing the thing'), being interviewed by Chris Morphet in 1972 and live video of three tracks. You can see some good close-ups of the mic and harness.
Well worth checking out - Search Vimeo - Duster Bennett Playing the Thing.
Not sure if the below link will work: