Oh that’s interesting. I always thought that was Waters sort of singing in character, but now it makes sense. I must admit I don’t know much about Roy Harper other than vaguely perceiving him as sort of a folkie. I know he’s mentioned in one of the song titles on Zep III.
Here’s Roy with Dave Gilmour... fantastic vocal work from Mr. Harper:
https://www.youtube.com/watch?v=xKjKL9fZayg