The jack for the headset earpieces is a quarter inch (6.35mm) in diameter, whereas the mic jack is smaller at 5.23mm. Being an electronics engineer in a past life I can only surmise that the connecting lead mentioned links the recorder to the quarter inch which has both the mic audio as 'sidetone' and the received audio to the earpieces.
Disclaimer - I'm no expert on aircraft electronics and the above is merely guesswork
