It would be lovely if this was authentic! However, there are some question marks. The Sutcliffe Estate has said it has no idea where this tape came from and who else is playing on it. All we can rely on is Pauline Sutcliffe's belief that it is Stuart's voice. Klaus Voormann and Pete Best, who must have heard Stuart sing this song many times, and likely a lot more often than Pauline, have already expressed that they don't believe it is Stuart. Everyone's now waiting on an opinion by Astrid Kirchherr.
What raises questions is the similarity to the rendition of the song in the 1979 movie `The Birth of the Beatles'. A lot of people believe that this might be an outtake from the production of the soundtrack for that film. The voices are highly similar, both songs are in the same key (different from Elvis' original) and the arrangement is pretty much the same. And that's the next problem: both versions seem to be based on Percy Sledge's cover version of Elvis' song, and that came out in... 1967! Years after Stuart's death.