The problem learning from Duck (whom I got to know quite well when I put on a local concert of his playing some years ago) is that he has very long fingers. So, he tends to anchor his thumb around the fretboard and move it up or down for bass lines. It's an interesting and subtle technique, but almost impossible for poor buggers like me with normal hands to copy! You can see him do this on some YouTube videos.

Having said that, his "Kid On The Mountain" CD from, I think, 1969, was for me the first and most exciting recording of Scottish and Irish tunes to be produced, and very influential. He played "The Blackbird" at the concert, and it was still stunning 40 years after he recorded it.