Best plan is to ask an engineer to look up the data/spec of the socket on the ATR, or e-mail ATR/hairbrush, they must have a tech query facility.
Single sockets are invariably (in my experience) NATO standard, and are more usually found in civvy helos rather than fix wing. Be aware though that there are 2 NATO single plug standards, European and US (Hey, great for commonality, that). It may or may not help that the US NATO plug, ie, the male piece on the end of the headset lead (again, in my experience) is always silver, while the Eu NATO is always brass/bronze. They are not compatible!
Just to complicate matters, there are different wiring protocols too - specifically whether or not Earths are combined or separated in the Tx/Rx sides of the circuit. Get it wrong and the headset won't work.
Suggest you repost this in Tech Log.
You'd be a sharp amateur electrician to make up an adaptor yourself, but it could be done if you were happy you had all the wiring specs and protocols to hand. You'll be the poorer by $$$ if you mis-identify the adaptor and buy the wrong one - but maybe the supplier will agree to refund/replace on return if not correct.
Good luck!