For more information there is a similar envelop with stamps at the Melbourne Museum : https://collections.museumsvictoria.com.au/items/1692627There were five issues in the 'Famous Australians' series released as follows: Issue 1, November, 1968; Issue 2, November, 1970; Issue 3, May. 1973; Issue 4, August, 1975; Issue 5, November, 1976.Have a look at the description from Melbourne Museum: https://collections.museumsvictoria.com.au/items/1692627stamps, first day cover, first day of issue, adam lindsay gordon, famous australians, lachlan macquarie, john duigan, reginald duigan, edward john eyre, philately

Colour photograph of the plaque on the memorial erected to the Duigan brothers first aeroplane flight. Plaque reads: 'This pylon commemorates the first flight of an aeroplane built and flown by an Australian about one mile east of this spot on 16th July 1910. John Robertson Duigan first flew the aeroplane which he and his brother Reginald built on their father`s property "Spring Plains". Unveiled by Air Marshal Sir Richard Williams KBE CB DSO 28th May 1960 Photograph taken on a BHS tour, possibly 1990's.aviation, civilian, duigan brothers