Frederic G. Mather says JS could read missing portions of papyri by "special revelation."
Frederic G. Mather, "Early Days of Mormonism," Lippincott's Magazine 26, no. 152 (August 1880): 211
Strange and dark cubbyholes stare at the visitor from all sides. In one of these was kept the body of Joseph, the son of Jacob, known by a roll of papyrus which was found in his hand. Joe Smith translated the characters on the roll, being favored with a "special revelation" whenever any of the characters were missing by reason of the mutilation of the roll.