Harvard University for decades assumed it had a cheap copy of the Magna Carta in its collection — a stained and faded document it had purchased for less than $30. But two researchers have concluded it is a rare version from 1300 issued by Britain’s King Edward I.

The original Magna Carta established in 1215 the principle that the king is subject to law, and it has formed the basis of constitutions globally. There are four copies of the original and, until now, there were believed to be only six copies of the 1300 version.