This article from the Jerusalem Post provides much of the history behind the unreleased track.
The original recording was undeveloped and needed work, which is part of the reason the release has taken so long. "I heard that George Harrison sings backup as well as playing at least one guitar," the source said. "It is thought McCartney sings at least some backup, if not more, and plays several instruments. It is also possible he wrote additional sections to the song."