A few weeks ago I attended a lecture at Drexel University here in Philly with David Lemieux (GD Archivist). I missed a good chunk of the talk because I couldn't get there in time but I wanted to share one cool story he told durring the q & a section.
The discussion got around to weather or not "everything" is accounted for, actually the original question I think was more to do with the band releasing rehearsals and sound checks ect. Anyway Lemieux responded with the story of the 'House Boat Tapes' and mentioned that they where sent to GDM after being found and where released as DP 35.
The really interesting story was that there is a show from 1979 that they don't have any tapes, aud or sbd to at all. Apparently the gig was somewhere in the midwest, and there was a huge blizzard keeping many people from attending. The band was already loaded in and evidently staying nearby the venue so they decided to play anyway. There was, according to Lemieux about 2000 people in attendance (and it was in a arena again don't know which one..). So nobody taped it, which apparently from the sbd end of things wasn't such a rare over site, Lemeiux went on to say that there where plenty of nights where nobody remembered to hit REC on the SBD deck, which now looking back is kinda tragic.
So there you go a "lost show" from as late as 1979.
I haven't googled it or anything, so if anyone knows more I'd be glad to hear about it.