The short, cut-down version of "Light My Fire" on AM was the "hit". The uncut album version was on underground radio, and was therefore "underground" music. All of Led Zeppelin's early albums were "underground" music.
I always preferred the album version of "Light My Fire" over the 45 edit. That also goes for "Beginnings" and "Ain't No Mountain High Enough" by Diana Ross. The radio edits are too short, so chopped up!