I have no idea, but just as a thought I'm just throwing in something in the lines of School Of Navigation RAAF/Australia/something. I'm taking liberties on the naming as it may well be that the acronym is nothing official but someones personal take of a unit/location/persons etc.
Where was the picture taken, do you know the name of the gentleman, would you be able to share the picture? There are plenty of very capable persons here to identify locations from WW2 era pictures, if that would solve the acronym.