Actually, Mark Zwonitzer is helpful on the copyright topic, too. Ralph Peer gave the Carters a set fee for each side recorded. But if they could copyright the song, they got royalties, too -- a half-cent for each record sale, and two cents for each sheet music copy. The way it worked was, A.P. owned the copyrights, but assigned them to Peer. According to Zwonitzer, Peer used this arrangement to keep the Carters wedded to Victor, and -- as Ron notes -- to motivate them to search out new songs. "If A.P. could continue to come up with new songs (or songs that could be 'put over' as new), Peer would make sure Victor sold them. The more songs the Carters could come up with, the better for them all."
The book is Mark Zwonitzer with Charles Hirshberg, WILL YOU MISS ME WHEN I'M GONE: THE CARTER FAMILY AND THEIR LEGACY IN AMERICAN MUSIC (2004)