The actual Line up included a 5th Band, a local Seattle Band by way of Wyoming named Butterfat, that opened for PG&E. Yes I was there with my buds (15-16 year-young-ens) enjoying a great afternoon into the late evening. Janis didn't get on stage (with bottle) until late ... after dark, So it wasn't over @ 10pm as shown on poster. (Seattle's sunset 9:15PM ish in early July)
This is the link to the boxing style poster listing the four featured bands ... Janis Joplin, Youngbloods, Steve Miller Band, Pacific Gas and Electric.
concertposterauction.com/detail.asp?id=5120 no longer exists
Here is the link to Butterfat band member Cory McDaniel. The concert is mentioned in the paragraph under his photo.
corypmcdaniel.com/music/cory-p-McDaniel no longer exists
Here is another reference to Butterfat in this blog talking about Quicksilver & NW Concets, scroll down to comment at May 30, 2010 at 03:26:21 AM
My brother went, but I didn't. Didn't make it to Satsop either. Did make it to see Butterfat, Pacific Gas and Electric (with Harvey Mandel), the Youngbloods, Steve Miller and Janis Joplin at Sicks Stadium. Sigh, a lost youth, I'm afraid. http://www.dailykos.com/story/2010/05/28/870682/-Popular-Culture-(Music)-The-Quicksilver-Messenger-Service-20100528
Thanks for letting me add my input to this great question!