Finally Carrouges stopped and began fumbling instead with the lock that held the visor shut. Le Gris, realizing the knight's aim, struggled all the harder. He rocked from side to side and wrenched his head around to thwart the attempt on the lock, all the while grasping uselessly in the sand for his sword.
Exciting stuff, but there's nothing like it in the historical documents describing the 1386 combat. Jager is guessing, and guessing badly. No surviving 14th c. helmet visor had a positive mechanical closure.
The good thing about Froissart was that even if Froissart invented details about a particular medieval deed of arms, he based them on things that he had actually seen.
That said, the end notes and bibliography in The Last Duel are a useful resource