As a big Tull (and Black Sabbath) fan, I'm not sure I ever knew this. Between playing in the bands Earth and Black Sabbath, Iommi tried out a stint as Jethro Tull's guitarist.
In this Ultimate Classic Rock piece, Tull's Ian Anderson talks about how he was willing to redirect the band's sound to accommodate Iommi's unique playing. But, after a month, Tony decided Jethro Tull was not for him and he returned to Earth, which would soon change their name to Black Sabbath.
Nothing was ever recorded with Iommi. He did appear with the band on The Rolling Stones Rock and Roll Circus (miming to the pre-recorded single for "Song for Jeffrey").
Image: Vertigo Records – Public Domain