The progressive rock trailblazers' fifth studio release was their first with producer Rodger Bain.
Paul McCartney called it a 'radical' release, but the song wasted little time becoming the group's seventh UK No.1 in two years.
Lewis Merenstein, whose most famous credit was producing Van Morrison's 1968 classic Astral Weeks, died on 6 September in New York at the of 81.