Written by Gianluca Mezzofiore, CNN

When an antiques dealer in Scotland bought an ivory chessman for £5 ($6) in 1964, he probably had no inkling that he had taken possession of one of the most famous chess pieces in the world.

Stored in a drawer for 55 years, the Lewis Warder, as the piece is known, could now fetch up to £1 million ($1.3 million) at auction after the late owner's family took it to Sotheby's auction house in London for assessment.

The chessman was recorded in the owner's ledger as 'Antique Walrus Tusk Warrior Chessman.' Credit: Courtesy of Sotheby's

The Lewis Chessmen were found on the Isle of Lewis in Scotland's Outer Hebrides in 1831, but the circumstances of their discovery are shrouded in mystery. With 93 pieces found -- the majority carved from walrus ivory -- the set was missing one knight and four "warders."

The leading theory about their origin is that they were carved between the late 12th and early 13th centuries in Trondheim, Norway, Sotheby's said in a press release.

Of the 93 pieces discovered in 1831, 82 are in the British Museum in London while 11 are in the collection of the National Museum of Scotland in Edinburgh.

The new discovery is a 3.5-inch warder, a bearded figure with a sword in his right hand and shield at his left side. In modern chess it would be the equivalent of a rook. It will be auctioned at Sotheby's London on July 2, the first time any of the Lewis Chessmen has been auctioned.

A spokesperson for the family, which wishes to remain anonymous, said in a statement that the warder was cataloged in the antiques dealer's purchase ledger as "Antique Walrus Tusk Warrior Chessman."

"From this description it can be assumed that he was unaware he had purchased an important historic artifact," the spokesperson said. "It was stored away in his home and then when my grandfather died my mother inherited the chess piece."

The Lewis Warder will be auctioned at Sotheby's in London on July 2. Credit: Tristan Fewings/Getty Images for Sotheby's

"My mother was very fond of the Chessman as she admired its intricacy and quirkiness. She believed that it was special and thought perhaps it could even have had some magical significance."

"For many years it resided in a drawer in her home where it had been carefully wrapped in a small bag. From time to time, she would remove the chess piece from the drawer in order to appreciate its uniqueness."

The Lewis Chessmen are "steeped in folklore, legend and the rich tradition of story-telling," Sotheby's said in a press release, adding that they are "an important symbol of European civilisation."

Alexander Kader, the Sotheby's expert who assessed the piece for the family, told CNN that seeing the chessman for the first time was a "delightful surprise."

"I said, 'Oh my goodness, it's one of the Lewis Chessmen,'" he recalled. "The family knew they had something special, but they were quite amazed."

"The new Lewis Warder has been on a remarkable journey to get where he is today," Kader said. "Over the last year we conducted a thorough study of him, undertaking many months of detailed research, art historical analysis, research into physical condition and provenance, and careful comparison with the other Lewis Chessmen."

In a press release, Kader said: "There is certainly more to the story of this warder still to be told, about his life over the last 188 years since he was separated from his fellow chessmen, and just as interesting, about the next chapter in his journey now that he has been rediscovered."