Stu refers to Ian Stewart, who played the piano.
He was also the piano player for the Rolling Stones, though he only appeared on albums, and not live, because management didn’t think he fit the image.

This was an informal jam session with Stewart on an out-of-tune piano, with Plant on acoustic guitar and Page on mandolin.

The song was never performed live.

Lyrics were nicked from Ritchie Valens Ooh
My Head, for which he sued in 1979.