Heather Carbo, a matter-of-fact librarian at an evangelical seminary outside Philadelphia, was cleaning out an archival cabinet one hot afternoon in July. It was a dirty and routine job. But there, on the bottom shelf, she stumbled across what may be one of the most important musicological finds in years.

It was a working manuscript score for a piano version of Beethoven's "Grosse Fuge," a monument of classical music. And it was in the composer's own hand, according to Sotheby's auction house. The 80-page manuscript in mainly brown ink - a furious scattering of notes across the page, with many changes and cross-outs, some so deep that the paper is punctured - dates from the final months of Beethoven's life.