THE SUIT OF ARMOR WAS UNUSUALLY small, in near-perfect condition, and appeared to be from the 15th century. But it wasn't until Paris antiques-dealer Pierre de Souzy let his daughter Diane, then 14 and a mere slip of a girl, try on his new acquisition that something clicked—especially after his wife, Mylène, now 45, jokingly compared her to Joan of Arc. "It was a revelation," recalls de Souzy, 50, who first thought the suit had been made for a child. "I immediately ran to my historical dictionary."

Since that day two years ago, de Souzy has had little else on his mind but the Maid of Orléans, who he believes was the original owner of the suit. "This quest [for proof] has become a passion," says de Souzy, whose shop now draws a stream of scholars and curiosity seekers. "I'm 95 percent sure it was hers."

The evidence is persuasive. Repairs on the 122-piece, 36-pound steel suit—which, if authentic, was commissioned by France's future King Charles VII in 1429—are consistent with injuries Joan suffered, and a portrait of the onetime shepherdess, painted some 50 years after her death, depicts strikingly similar armor. Also, the Paris dealer from whom de Souzy bought the suit (he won't say for how much) claims it came to France 200 years ago from England, the country whose invading citizens burned Joan of Arc—and presumably kept her possessions—as a heretic in 1431. "If it's a fake," says Jean-Pierre Duchiron, an antique-arms expert, "it comes from a phenomenal workshop."

To be sure, skeptics abound. Olivier Bouzy of the Centre Jeanne d'Arc, an archive in Orléans, claims the style of the armor's tassets—two pieces on the upper thighs—postdate the heroine by 20 to 30 years. But de Souzy—who has no plans to sell the suit—isn't discouraged. "I'm doing this for history," he says of his ongoing search for proof. "It's the most fabulous object I could ever dream of finding."

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