For what it's worth, the Grateful Dead Day by Day site indicates that the Dead never played Europe in August of any year so it's unlikely it ever took place. They did play three shows there in Oct. '81, that would be about the same era that Sound Machine existed. Perhaps that tour was originally scheduled for August?
Do you know if the posters's authentic? If you don't know do you have any history of the poster (where it came from, etc.)? Do you have a scan you could email me (I might put it up on my GD concert poster site)? Iver
