Introduction: June 1967 - December 1968 1 January - August 1969 2 August 1969 3 August - December 1969 4 January - April 1970 5 April - October 1970 6 October - November 1970 7 December 1970 - May 1971 8 June - December 1971 9 January - April 1972 10 April - September 1972 11 September 1972 - June 1973 12 June - November 1973 13 November 1973 - June 1974 14 June 1974 - December 1976 15 January - December 1977