Ta Prue - for the slouch hat info, my rellie is not disagreeing about when the slouch was turned up by the wearers, but is of the view that the "authorities" only changed the printed rules for turning up the brim because of the losses (KIA and injured) at Fromelles. She recalls her forebears explaining that studios would not take photographs of men where their uniforms were not fully complying with the written rules, for fear of the studios being denied access to the military camps. She also mentioned that there may have been debate about which side to turn up, the side for the salute (right hand side) or the side for the bayonet (left hand side).
As this photo was taken by a studio at the military camp at Liverpool (a suburb of Sydney NSW), then perhaps the date that those written rules were charged could help to date and thus limit the number of candidates to help find the correct David Brown. Twas just a thought though.
My rellie's forebears had photographic studios from Degurotype times until post WW2 (I'm not sure if "Degurotype" is correct spelling, but it sounds like that, - I'm only relaying phone conversations from an elderly rellie who is relying on her own childhood memories).
I have also found a website re the Light Horse, perhaps Wallytat may find it helpful in this search as there is a forum there.
http://www.lighthorse.org.au/ The following link shows the various uniforms worn by AIF service men and women during WW1.
http://www.sparkefilms.com/wardrobe/ww1.htmlCheers, JM