This great answer came from Chris Read in response to my query to the State Library of South Australia:
"the Library has many Duryea portrait photographs, and I have checked the archives to view a number of the original photographs. Not many of them actually had Duryea studio numbers written on them, however I did find Nos. 2824, 4043, 15008, 45453 & 56780. None of these had dates on them, although the Library has attributed approx. dates circa 1865-1870. We may have received information from donors.
However, there is an excellent website called the History of Photography in South Australia, put together by the Art Gallery of SA following a photographic exhibition a few years ago. Part of this website details the Duryea numbering system and dating the Cartes de Visite. I imagine you will find this very helpful, and probably dates your own photograph to circa 1867-68.
http://www.artgallery.sa.gov.au/noye/Dating/Dat_set.htm
An interesting book is Julie Robinson's "A Century in Focus: South Australian
photography 1840s - 1940s" published in 2007 which we have here at the Library. This
was published in conjunction with the Art Gallery's exhibition."
Thanks Chris! And everyone who led me in that direction.