Dauger (or whatever his name truly was) died in 1703, which, as keen-eyed readers may notice, means he sat in the Bastille with his head in a fucking crock pot for four dick-punching years. The subsequent 300 years have done nothing to uncover the mystery of his identity, either -- there were few solid facts about the man to begin with, and three centuries of retellings laced with spirited embellishments have seriously diluted what little we had to go on.
At least they permitted him the dignity of shoe and knee bows.
The popular theory is that he was some kind of high-level political prisoner, hence the need to conceal his identity. However, nobody can quite agree on exactly who that famous inmate might have been, with some of the wilder guesses ranging from Oliver Cromwell's son to the older brother of King Louis XIV himself (Dumas' novelization of the DiCaprio movie is based on that second idea). All we really know about him is from letters written by the Bastille's governor and from his fellow prisoners spreading the story, and those testimonies are contentious about whether his mask was even made of iron to begin with. To be fair, "The Man in the Black Velvet Vanity Veil" doesn't carry quite the same level of mystery and intrigue.