The Supremes — a cornerstone of Motown’s success in the world of popular music in the 1960s — were not doing quite so well in 1968. Their primary songwriters had departed the label, leaving the trio with declining record sales and radio airtime. One of the Supremes found herself replaced. And lead singer Diana Ross was clearly contemplating setting out on a solo career. A crash project put in motion by Motown owner Berry Gordy resulted in a most unusual song, released 55 years ago Saturday. It would become the Supremes’ 11th No. 1 single.