I could not find anything about headsets in the VC10 OM, but the 'interphone' subchapter included this bit:
INTERPHONE
CREW I/C
Depress I/C button on Station box for reception, adjust volume by turning button. Check that the correct technique is selected on the microphone selector panel (BOOM - EM., MASK - CARBON) .
Set I/C - RT switch to I/C and speak.
So apparently it was a headset with an electret microphone. The plug does appear to fit the connection boxes on the VC10 flightdeck, but that's just my guess.