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.
