Last year, the Czech musicologist Petr Danek was in the Franciscan library in Slany, about 20 miles northwest of Prague, searching for Renaissance and early Baroque musical prints. He noticed large bound volume on a high shelf and, taking it down, realized it was a special find: the bass parts for a rare collection of vocal polyphony from the beginning of the 17th century.

“But then I suddenly came across something marked ‘Violino,’ ” he recalled in a recent interview. “I turned the page, and I immediately knew what it was.”

Mr. Danek muttered a Czech epithet. He had found the missing ritornellos of “The Czech Lute,” one of the great works of the Czech Baroque.