CHEVALIER D’ÉON, (Charles Geneviéve Louise Auguste André Timothee d’Éon de Beaumont) born (d: 1810); A French diplomat, spy, soldier and Freemason who lived the first half of her life as a man and the second half as a woman. If you look up eonism in your dictionary, you may or may not find it. It’s a pseudo-scientific word for transvestism that has the quaint ring of Stekel or other Herr-Doktors who were writing in turn-of-the-century Vienna when people seem to have been having love affairs with their piccolos or putting clown hats on their penises.

The Chevalier d’Eon, a century before, loved to dress in drag. Supposedly, the French adventurer originally had been sent on various spying missions disguised as a woman and so liked the feel of silk and satin that he decided to stay dressed in costume the rest of his life.

Not that he had much choice. When word got out that he really liked taking four hours to dress each morning, King Louis XV decreed that the chevalier had to wear women’s clothes forever after. For years people laid bets on d’Eon’s sex, but a post mortem examination of his body conclusively established the fact that he was very much a man.

Charles Geneviève Louis Auguste André Timothée d’Éon de Beaumont, to give her full name, is one of the most important transvestites in history. She was “a fascinating and inspirational figure”, said Lucy Peltz, the London National Portrait Gallery’s curator of 18th-century portraits.

The painting in the illustration was discovered by a London dealer at a provincial sale outside New York. It was being mistakenly sold as a portrait of an unknown woman by Gilbert Stuart, most famous for painting George Washington on the dollar bill.

“Even in its dirty state it was quite clear that this woman had stubble,” said Mould, who bought it, brought it to the UK and began further research and restoration.

“Cleaning is always a revelation and on this occasion it revealed that not only was it in lovely condition but, more pertinently, the Gilbert Stuart signature cleaned off revealing the name Thomas Stewart, a theatrical painter working in London in the 1780s and 1790s.”

Everything then began to click into place. “What is so unusual about this portrait is that it is so brazenly demonstrative in a period when you don’t normally get that type of alternative persona expressed in portraiture,” said Mould. There is no attempt to soften his physiognomy – basically, he was a bloke in a dress with a hat.”

Even without the cross dressing D’Eon is a seriously interesting person. Before living publicly as a woman he was a famous French soldier and diplomat who had a key role in negotiating the Peace of Paris in 1763, ending the Seven Years War between France and Britain.