Pierre Henry is mentioned in this article. JMJ says he met him while at the GRM. First contemporaneous article I've seen that mentions Pierre Henry and not just Pierre Schaeffer.
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01.09.93 Manchester / 07.06.97 Manchester / 30.03.08 London / 22.05.09 London / 10.10.10 London / 01.07.11 Monaco / 04.05.16 London / 23.07.16 Macclesfield / 06.10.16 Brighton / 07.10.16 London.