On July 13, defense lawyers produced 500 pages of documents, 200 of which they said were mistakenly omitted earlier in the litigation, according to a court filing. The new documents included a 1927 publication of the song lyrics that included no claim of copyright. Further investigation by Ms. Nelson’s lawyers uncovered an edition of the book from 1922, also with no copyright notice.

The court filing argues that copyright law at the time required such a notice and that without it the work was “interjected irrevocably into the public domain.”

Image Birthday parties have remained free of copyright claims over the song “Happy Birthday to You.” But its use in movies, television and radio has required licensing fees. Credit... Todd Heisler/The New York Times

Mark C. Rifkin, one of Ms. Nelson’s lawyers, said they brought the new evidence to the court’s attention within hours of its discovery. A hearing is scheduled for Wednesday in Los Angeles.

A spokesman for Warner/Chappell did not return a call seeking comment. In 1988, the company paid about $25 million to acquire Birchtree Ltd., which owned the song.