My view is that documentary evidence of the following kind is VERY strong: * ticket stub * concert poster * flyer * newspaper ad * newspaper review. Not invallible because tour plans sometimes change, but extremely strong nonetheless. The balance of probability here is strongly in favour of Cologne. Ticket has not been used which is a small negative, but on the other hand there is absolutely no contemporary documentary evidence for Bonn; without such there is really no contest.
A ticket may remain unused for many reasons. I've booked a few concerts over the years which in the end I have been unable to attend. I would assume the gig went ahead as planned without substantial evidence to the contrary. As often the case, the best we can do is assess the balance of probability, then re-evaluate if counter evidence comes to light.
I'm also leaning towards Cologne. A ticket, the "Musik-Express" magazine and a poster seem to be more proof than a mention on roio cover art. The fact that the ticket is unused doesn't really proof anything, because if the concert would have been moved from Cologne to Bonn, the original ticket (stating Cologne) would probably still have been valid for the Bonn show.
So let's just assume Cologne is correct. (I might need to update my database again) :)
What I would like to know is where "Bonn, Rheinterassen" came from. Now, we only have the cover art, created by mc-merc (whose artwork is always excellent, imo!) who must have taken the info from the accompanying info file. So, the owner of the recording must have had this venue listed on file or something. I searched but couldn't find another Camel concert held in Bonn (so that the dates could have been mixed up). Maybe the owner of the tape was confusing different recordings by different bands?
Would that be a venue where Camel would play? Or would it have been another venue with the same name (Rheinterrassen seems a generic name to me, maybe the band just played somewhere at the border of the Rhine).
to Longtalleric: I do not think that was meant by "Rheinterassen" that hotel.
You know how great I posters and tickets from concerts Camel find the forum and to show my home page. Not that I do not believe you, but there is nothing better than such discus ions with original documents to support.