The Kingston Trio were able to copywright their folk song-based material by the addition of a new verse, the changing of a line or some key words, etc., and altering the music slightly. My understanding is that this was to prevent anyone from singing their personalized version of the song without compensation through ASCAP or BMI. It was their only way to protect themselves since most of their older stuff came from the public domain. I'm sure the agents for most folk groups of the time used the same technique.