On the studio tracks, it would have either been Sterl, or Doug overdubbing the bass afterwards. You can normally work it out from whether the track has one or two guitars. Same for the live stuff I guess. They may have done some numbers without bass.
It reminds me the May'70 inthe Second Fret,Philadelphia where they gave
the same of gig (although the mood was darker) with Doug switch from bass to
drums and even guitar
I really like those show a lot