This microphone plug 3/16" or .206" is also used with Collins gear and avionics microphones.
Switchcraft makes them. Many distributors sell them, including Newark Electronics. They are avaialble every week on eBay at reasonable prices.
Surplus Sales of Nebraska sells genuine Collins tagged PJ-608 (expensive tag).http://www.surplussales.com/Collins/CollMisc-1.html