This is a rather tricky question. As detailed in "In a Nutshell," Dumas never wrote a novel entitled The Man in the Iron Mask. Instead, he wrote a super-long novel entitled The Vicomte de Bragelonne. For ease of reading, translators typically break the novel up into several parts, the last part being The Man in the Iron Mask.
In our opinion, those translators didn't read the book very carefully. Yes, readers do encounter a man who wears an iron mask, but he is a minor figure in the novel. Can you think of any possible titles that would be more appropriate?