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In Adelaide, a slouch hat marked “75” on a man born in 1900 almost certainly points to the 75th Infantry (SA) cadet or militia unit, not WWI overseas service.

Most likely it’s his Citizen Forces/Senior Cadets unit number—specifically the 75th Infantry (South Australia).

In South Australia, the 1911–1929 compulsory training scheme put boys/young men into Senior Cadets (ages ~14–18) and then the Citizen Forces/Militia. These wore slouch hats very similar to AIF soldiers, often with the unit number on the hat/puggaree. 

 

By 1912 SA had nine infantry regiments numbered 74–82, which includes the 75th Infantry; RSAR records list “75th Infantry (1911–1919).” Your grandfather (born 1900) would have turned 18 in 1918—exactly the age to be in those cadets/Citizen Forces in Adelaide.