"It's hugely exciting and there's an awful lot at stake here," said David Tripp, a coin expert and the author of the book Illegal Tender: Gold, Greed and the Mystery of the Lost 1933 Double Eagle, which tells the story of the $20 coin and a remarkable 70-year operation by the US secret service to recover the handful that were not melted down to bullion.
"If the government prevails, the value of this one coin increases immensely. But if the family prevails, and there are another 10 of these coins on the market, their value goes from almost $8m <£4.4m> to about $800,000 each overnight."
Mr Fenton was arrested when he tried to sell his coin, once owned by King Farouk of Egypt, to undercover secret service agents at a New York hotel. The subsequent auction settled his legal dispute with American authorities.
Mrs Langbord's father, the Philadelphia jeweller Israel Switt, was a leading suspect in the "theft" of a number of Double Eagles from the mint in 1937, when the 445,500 coins minted three years earlier were recalled and destroyed.
Roosevelt's ruling meant that the coins were never put in circulation, adding to their value. "It was a wonderful paradox, where the coins were legally made by the US government but illegally owned the second they were pressed," Mr Tripp said.
Switt sold nine to private collectors over the next decade, before he was persuaded that the sales were illegal and ordered to forfeit the coins still in his possession.
David Lebryk, the acting director of the US mint, said Switt had been involved "in a variety of questionable activities" but had never been charged with a crime.
http://www.guardian.co.uk/international/story/0,3604,1557319,00.html