The first time Grigory Kessel held the ancient manuscript, its animal-hide pages more than 1,000 years old, it seemed oddly familiar.

A Syriac scholar at Philipps University in Marburg, Germany, Dr. Kessel was sitting in the library of the manuscript’s owner, a wealthy collector of rare scientific material in Baltimore. At that moment, Dr. Kessel realized that just three weeks earlier, in a library at Harvard University, he had seen a single orphaned page that was too similar to these pages to be coincidence.

The manuscript he held contained a hidden translation of an ancient, influential medical text by Galen of Pergamon, a Greco-Roman physician and philosopher who died in 200 A.D. It was missing pages and Dr. Kessel was suddenly convinced one of them was in Boston.