Map sales have become a hot topic on social media, but an antiques dealer found a truly historic item among the famous couple’s possessions.

On a virtual tour of Christie’s for the auction organized by the foundation that manages the estate of the heir to oil magnate Gordon Getty and his wife Anne, Alex Clausen of Barry Lawrence Ruderman Antique Maps Inc noticed an old nautical chart known as a portolano .

Christie’s had dated the map to between 1500 and 1525 with an estimated value of between $100,000 and $150,000.

Getty and his wife had also bought it from Christie’s during an auction in 1993. The couple had the map restored and for years it hung in the library of their San Francisco mansion. They then paid about 56,500 British pounds for the map, about $85,000.

Clausen, president of Barry Lawrence Ruderman Antique Maps Inc., which scours the websites of auctions, estate sales and other dealers in search of antique maps, manuscripts and important historical documents that have been forgotten or ignored, sometimes for decades, suspected it was much older. In October last year, he and his team bought the map from Christie’s for $239,400.

After months of research—including pigment analysis, multispectral imaging, and consultation with scholars—Clauzen and his colleagues determined that the parchment map was likely created in Venice around 1360.

The new date makes the Getty Foundation’s acquisition the oldest portola map in the U.S.—predating those owned by business titans and portola hunters Henry Huntington, J. P. Morgan or in Museums and Libraries.

Alex Clausen emphasized, according to a report in the Los Angeles Times, that his find is worth $7.5 million.

The map is the only complete 14th-century portola known to exist outside Europe and a very important element in understanding the development of European cartography in the Mediterranean, said Richard Flederer, an independent academic specializing in the field.