If there was a kind of distilled-down essence-of-Blues, this might very well be it:
From the mid-Sixties, on tour (prob. in the UK) as part of the American Folk-Blues Festival.
About death, of course. Leaning over to sound the bass strings, like the tolling of the death knell. "Everybody dies - dies with the heart trouble. Cos when that tick-tock stops ticking, you don't keep time no more".
An extraordinary performance.