An original copy of the Magna Carta has been found in a Harvard University library, stunning British academics who hailed the school’s previously perceived imitation as “one of the world’s most valuable documents.”

The historic charter, which dates back to England’s King Edward I in 1300, was declared an original by eagle-eyed historians in the U.K. who launched an investigation into its authenticity after coming upon it online, Harvard University said in a release Thursday.

Nicholas Vincent, a professor of medieval history at the University of East Anglia, said it took “all of 30 milliseconds” for him to recognize the document’s authenticity after it was flagged to him by David Carpenter, a fellow professor of medieval history at King’s College London.

“You know what that is and I know what that is, it’s an original Magna Carta,” Vincent recalled his immediate reaction to Carpenter by email.

The New England school said it purchased the roughly 700-year-old document in 1946 for $27.50 (approximately $500 today) at auction. The auctioneer, who obtained it through a London bookseller who acquired it from a Royal Air Force war hero, misidentified it as an unofficial copy after misreading and labeling its date as 1327, Harvard said.

There are only 25 known original Magna Cartas today, including this one owned by Harvard. This new one is the third known outside of the British Isles — the other two being in Washington, D.C., and in Canberra, Australia, according to the university.

The document was first issued by King John, King Edward I’s grandfather, in 1215. It was the first to put into writing the principle that the king and his government are not above the law. Amended versions of it were released in years after.

“It is an icon both of the Western political tradition and of constitutional law. If you asked anybody what the most famous single document in the history of the world is, they would probably name Magna Carta,” said Vincent.

The badly faded and stained document had been archived in Harvard Law School’s library since its purchase. A close review of it by Carpenter and Vincent found that it perfectly matched six other original copies from King Edward’s 1300 confirmation of the charter that are known to still exist. It notably matched the dimensions and handwriting, and it also contained some small changes that those original six copies have from earlier versions.

“This is a fantastic discovery,” Carpenter said. “Harvard’s Magna Carta deserves celebration, not as some mere copy, stained and faded, but as an original of one of the most significant documents in world constitutional history, a corner stone of freedoms past, present and yet to be won.”

Those wanting to view the document can see it digitized at the Harvard Law School Library’s website.