Erika Delbecque, a special collections librarian at the University of Reading, was cataloging a box of items in the university’s archives when she noticed something unusual: a visibly old, double-sided leaf of paper stamped with blackletter typeface and red paragraph marks. Delbecque immediately realized that these were hallmark signs of the earliest western European printing, and experts have confirmed that the text is indeed an incredibly rare find. As Rachael Revesz reports for The Independent, the pages once belonged to one of the first books printed in England.

Written in Medieval Latin, the pages date to late 1476 or early 1477. They come from a religious handbook called the Sarum Ordinal or Pye, which instructed priests on how to prioritize feast days for English saints. According to a University of Reading press release, the handbook once contained around 160 leaves and was based on an 11th-century manuscript by St. Osmund, the Bishop of Salisbury.

Experts say that the text was among the first books printed by William Caxton, a pioneering English publisher. The Encyclopaedia Britannica writes that Caxton was a wealthy trader who established his own printing press and translated an epic romance called Recuyell of the Historyes of Troye, which was printed likely in late 1473 or early in 1474. Caxton would go on to print a large number of service books and devotional texts—the Sarum Ordinal among them.

The newly discovered leaf is one of just two surviving fragments from this medieval handbook; the other, which consists of eight double-sided leaves, is housed in the British Library in London. Delbecque says that the leaf “had previously been pasted into another book for the undignified purpose of reinforcing its spine,” according to the press release. A librarian at the University of Cambridge rescued the leaf from its ignoble fate in 1820, but did not seem to have realized that the text was a Caxton original.

The University of Reading unwittingly purchased the leaf in 1997, when the institution acquired a vast collection that belonged to the late typographer John Lewis. For 20 years, the Sarum Ordinal leaf sat, unnoticed, in a box filled with thousands of items—until Delbecque’s keen eye recognized the text as a bona fide historical treasure.

“It is incredibly rare to find an unknown Caxton leaf,” Delbecque says, “and astonishing that it has been under our noses for so long.”

After decades spent in obscurity, the pages are getting some time in the spotlight. They will be displayed in the University of Reading’s special collections department until May 30.