According to Tom's web page "It's an open D: starting with the bass string - D,A,D,F#,A,D. Back when I recorded it I seemed to be tuning my guitars down either a half or sometimes a whole step from A=440, so the recording may be pitched lower than D."

In his early years Tom didn't play normal slide guitar with a typical finger slide, although perhaps he does now. He put the guitar face up on his lap and played itsort of of dobro style and often using a common table knife handle as a slide.   He counsels against using a sharp knife.