IN THE FALL of 1974, a small number of groups had multiple album entries lodged firmly in the Top 200 chart of Billboard Magazine, the bible of the recording industry. Seals and Crofts had three contenders. So did such heavies as Elton John, Jim Croce, Bachman-Turner Overdrive, John Denver and Stevie Wonder. Only one group, though, was represented by more plums than that, and that group was light-years beyond the pack. Their five-year career was composed of seven albums, and all seven are still floating in the Top 200 with unparallelled immortality; five of those seven are among the 150 best-selling albums in America.