Few people know that killer riff was actually the work of Duanne Allman, he was a session guitarist at the time in the studio and was brought in by the owner of the studio to help out, Clapton and friends were stumped and couldn't quite get the feel they wanted on "Layla", if you listen close you will realize that
slide sound shows up later in all the Allman Brothers albums, that bottle neck slide is unmistakenable.