This is interesting...
Dietze Joseph Henry [Sandoe Joseph Henry] : SERN Second Lieutenant 94 : POB Cornwall England : POE Sydney NSW : NOK F Dietze Robert
Written in red is SANDOE - True name. He was 20 and 8 months when he enlisted in Aug 1914 (though in Feb 1918 he is stated to be 24 and 7 months!) He was an engineer, Wesleyan...dark hair, eyes and complexion. Wounded in Gallipoli, then KIA Sept 1918.
The army advised that when a solider serves under an assumed name it is practice to put both names on the headstone, but his mother expressly requested that only Dietze be inscribed 'which name he adopted of his own free will'. There is also a letter written by Robert Dietze stating that Joseph is his stepson and son of his wife's first marriage.
Looks like he was born before his parents married?
Joseph Henry SANDOE's birth registered LISKEARD, CORNWALL March quarter 1894