My first hide down on the plain!
In 1968, I arrived in Adelaide from England with my Ten Pound Pom father and Australian mother. For the first two years we lived with my mother's mother in Kilkenny on David Terrace while my parents found their feet, and built a home in Modbury. I don't recall any trips to post offices in those days. Arndale had already been built, so I imagine the nearest post office was here, though possibly not in this current location.
At the listed coordinates you are standing in front of a bank of post boxes.
Let ABC be the number of the box to the left of #338
Let DEF be the number of the box to the right of #457
The cache is located a short walk away at:
S 34º 52.(F - B)(A - B - C)(D - A) E 138º 32.(B + E)(F - D - B)(A + C + D) Checksum 23.
Have fun!