"Not necessarily. TC did some work in the studio with the GD before joining the band in December 1968."
Yes, but that was before St Stephen had been written, let alone paired with the Eleven. This rehearsal could not be from, say, March or June '68. Not only am I sure this is TC and not Pigpen, I think the style of Stephen (less frenetic than in the summer) makes clear that it's from later in the year. How late is the question... It is quite possible that TC used some leave time to do some rehearsing with the Dead in fall '68, knowing he'd be joining them upon discharge. "Pacific High was at 60 Brady Street... The studio in San Mateo was called Pacific, but it wasn't Pacific High." You're right - caught me there! I was thrown off partly because the best circulating copy was labeled "Pacific High" - but, like most of the labels for this recording, that seems to be false. Blair Jackson says the Dead didn't move to Pacific High until 1969.
