Many thanks. Found partial answer in ICAO Annex 11,
Amendment 23: use of SSR code 7500 in the event of unlawful interference; requirements for communications between ATS units and meteorological offices. Adopted/approved 12 December 1975; effective 12 April 1976; applicable 12 August 1976.
So 7500 was in use by December 1975; but it's not clear when it came into effect for communications between flight crew and ATS.