From the blog of somebody named Jean-Claude, we have a link to one of his photos. (Thanks Olvator).
Jean-Claude labeled this "1971? arenes du Grau du Roi". Any more info would be appreciated. Ork Alarm has the following information listed about a venue called Grau du Roi.
1970-09-05 Le Grau-du-Roi, "Belin Biscuits" Contest
There is also another photo from Jean-Claude from this concert. It's a photo of Jean-Luc Ponty, and the caption says that JLP was a guest of Magma at this concert.
Update: Our friend Akoutikus sends proof of this concert in the two scans below:
Caballo de Feria - Diluvio
6 years ago