It is a sad story really. It is thought that he may have been forced to wear the mask for up to 42 years, even at death in 1703. Although speculation and gossip that he was very important, and well-dressed, and the prison governor called him �my prince� was rife, it was Voltaire who in the 1720�s suggested that he looked like someone very famous, the idea that it was the twin of the King was one put forward by a journalist in 1789 (86 years after his death). A story elaborated and perpetuated by Dumas in the 19th C.
As said above, it is unlikely we will ever know. But I reckon truth and fact gave way to idle gossip and speculation, and at the time nothing could be more fascinating than the prisoner being a contesting heir to the throne of France. One would assume if this really were the case then they would have been put to death, not held captive for so many years. I find it more likely that the handsome dandy was perhaps a contesting beau in the court of Versailles and the King was jealous of his looks or his affairs.