- Writer and philosopher Voltaire claimed in the second edition of his Questions sur l'Encyclopédie (published in 1771) that the prisoner wore a mask made of iron rather than of cloth, and that he was the older, illegitimate brother of Louis XIV. In the late 1840s, writer Alexandre Dumas elaborated on the story in the novel The Vicomte of Bragelonne: Ten Years Later, the final installment of his D'Artagnan saga: here the prisoner is forced to wear an iron mask and is Louis XIV's identical twin. Dumas also presented a review of the popular theories about the prisoner extant in his time in the chapter "L'homme au masque de fer" in the sixth volume of his Crimes Célèbres.
What little is known about the historical Man in the Iron Mask is based mainly on correspondence between Saint-Mars and his superiors in Paris.
The National Archives of France has made available (online) the original data relating to the inventories of the goods and papers of Saint-Mars (one inventory, of 64 pages, was drawn up at the Bastille in 1708; the other, of 68 pages, at the citadel of Sainte-Marguerite in 1691). These documents have been sought in vain for more than a century and were thought to have been lost. They were discovered in 2015, among the 100 million documents of the Minutier central des notaires. They show that some of the 800 documents in the possession of the jailer Saint-Mars were analysed after his death. These documents confirm the reputed avarice of Saint-Mars, who appears to have diverted the funds paid by the king Louis XIV for the prisoner. They also give a description of a cell occupied by the masked prisoner, which contained only a sleeping mat and (as was previously thought) no luxuries.