"Stu" is Ian Stewart, the Rolling Stones road manager and piano player. The song was recorded when Led Zeppelin was set up in Headley Grange. They were using the Rolling Stones mobile recording unit and Stu stopped in. There was an old piano in the main room that was slightly out of tune. Stu was playing it and Jimmy Page was tuning his guitar to how the piano was out of tune. Tape was rolling and everything was pretty much improvised from Bonzo's drumming to the lyrics Plant was singing.
Joel - Bowie, VA