He played bass more often than people realise, and even owned a bass or two himself!
There are quite a few instances on record (as in "documented fact") of him standing in for bass players, or picking up the bass in a jam.
Here are a couple of photographs of a gig which he played with Johnny Winter - and to make it even more unusual, Johnny seems to be playing a Stratocaster, rather than his trademark Firebirds!
Here he is with Noel Redding's beautiful old sunburst Jazz -
And in the studio, with a P bass -