When Rob Langbord's mother asked him to check a long-neglected family safety deposit box, he had no clue as to the treasure he would find...

According to a report in the New York Times, he found an incredibly rare coin, wrapped in a delicate paper sleeve.

It was a gold $20 piece with Lady Liberty on one side, a bald eagle flying across the other - and the date read 1933.

The famous 1933 "double eagles" were never officially released by the government.

Only a few had ever made their way out of federal vaults, and only one ever sold publicly, in 2002, for $7.6m.

In Langbord's deposit box, there were a total of ten 1933 double eagle coins.

But, when the family took the coins to be authenticated at the United States Mint in 2004, they were in for an unpleasant surprise.

The Mint identified the coins as genuine, and kept them.

The coins, claimed the mint, were government property stolen from the Mint, most likely in the 1930s, by Mr Langbord's grandfather, Israel Switt, a Philadelphia jewellery dealer.

The Langbords went to court and their lawyer, Barry H. Berke, recently won an important ruling - a US District Court judge has given the government until the end of the month either to give back the coins or go back to court to prove that they were in fact stolen by Mr Switt.

That could be a daunting task after three quarters of a century.

Mr Langbord's grandfather Israel Switt was the dealer who first procured the coins.

The Secret Service, which polices currency crimes, has argued that all of the double eagles that escaped government control passed through the hands of Mr Switt, working with a corrupt cashier at the Mint.

A Mint spokesman declined to comment on the case because of the litigation.

The Langbords insist that Mr Switt, who died in 1990, acquired the coins legitimately before the ban, most likely through a gold-for-gold exchange process used by the Mint in those days.

In fact, said Mr Berke, Mr Switt was a frequent visitor to the Mint - he had plenty of gold to trade for gold coins, and probably did so, and proving otherwise will be extremely difficult.

"There's really nobody with contemporary knowledge from the Mint who's still living," he said, having sought out former Mint employees for the case.

Due to Mr Berke's intervention, the government now carry the burden of 'proving a negative' regarding Mr Switt's conduct.

"Nobody can prove conclusively what happened. Anybody who has to do that, I think, is going to fail," said Armen Vartian, the general counsel of the Professional Numismatics Guild.

Mr Vartian said he would be happy if the Langbords were allowed to sell the coins.

"Maybe they were stolen in the 1930s," he said, "but they certainly weren't stolen by the people who are holding them now," he said.

The 1933 gold double eagle: a coin with 'so much charisma'

Nearly a half-million 1933 double eagles were minted before President Franklin D. Roosevelt issued an executive order that made owning large amounts of gold bullion, and coins, illegal.

Two of the coins went to the Smithsonian Institution, and almost all the rest were melted down.

Some coins, however, escaped the melting. Once such coin was sold in 2002 and ended up in the collection of King Farouk of Egypt, then later the hands of a British dealer.

The government seized that coin, too, and arrested the dealer, Stephen Fenton.

But Mr Fenton's lawyer reached a settlement allowing that single coin to be issued officially and sold at auction, with the government taking half of the proceeds.

Mr Fenton advises selling just one or two of the coins a year.

With the right timing and a good market, he said, they could bring $4m to $6m each, because there are many people who would want to own one.

"This coin," he said, "has got so much charisma."