A rare silver dollar dating from 1866, perhaps the world’s most notable missing coin, might have been turned over to experts last week in the bar of a Best Western hotel in Maine.

The coin, valued at more than $1 million, was surrendered in Augusta by a librarian who said he received it from an eccentric friend several years ago in a box of coins of considerably less value.

Only two 1866 silver dollars without the inscription “In God We Trust” are known to exist. The other is in a private collection, and this one was largely written off as lost after gunmen snatched it in 1967 from millionaire Willis H. du Pont, to whom it will revert.

“The darn thing turned up,” said John J. Kraljevich Jr. of the auction firm American Numismatic Rarities. He and a colleague, John M. Pack, declared the coin to be the real thing after huddling with the librarian at a table in the Best Western last week.

Experts from Numismatic Guaranty Corp. will examine the coin for authenticity in Baltimore next week, in advance of a coin show there.

Kraljevich pegged its value at $1.5 million. Richard Doty, curator of the Smithsonian’s National Numismatic Collection, estimated it Monday at “rather more than $6 million.”

The so-called 1866 “no motto” silver dollar occupies a rare position in the lore of coin collectors, in part because of its scarcity and in part because of the daring heist during which it was stolen.

“This is a very big deal,” said Christopher Cipoletti, executive director of the American Numismatic Association, who said the coin is worth “well over a million.”

In October 1967, hooded gunmen broke into du Pont’s mansion in Coconut Grove, Fla., after midnight.

They tied up the chemical magnate and his wife, ransacked the house for more than two hours and made off with more than 7,000 rare coins, a collection then valued at $1.5 million, according to contemporary news accounts.

The gunmen bound the du Ponts with neckties and Venetian blind cords and then dined on the millionaire’s roast pork and chided him for not “working to earn a living like everybody else.”

Du Pont, who over the decades has taken great interest in the effort to recover the stolen coins, was “elated” at the news, his attorney said Monday.

“We’ll be thrilled to death if that’s the coin, and we’re fairly certain it is, but we’re going to withhold our judgments until it’s examined at the show in Baltimore,” attorney Harold Gray said.

The heist remains unsolved, and much of the collection has not been recovered. Gray said he keeps the FBI posted on developments involving the collection.

Gray has traveled widely to recover pieces of the stolen collection, including to Switzerland, Germany, London and Italy.

“We recovered 13 territorial gold coins, very unique and valuable pieces, from the mob in Philadelphia,” he said.

The path that the 1866 coin took from du Pont’s walk-in safe to a bank in New Hampshire, where American Numismatic Rarities is based, remains murky.

The librarian, whose identity Kraljevich would not reveal, told Kraljevich he took it as collateral for a loan he gave to a friend who died in 2001.

He contacted Kraljevich after the firm offered the sister coin at auction in the fall.

They met in Augusta on Thursday. Kraljevich said they sat in a quiet corner with an outlet for a lamp and scale. It took very little examining, Kraljevich and Pack said.

“Even a quick glance at this you could tell it was perfectly genuine, and this was the piece we’d been looking for for years,” Pack said. “Everything was right about it.”