Welcome to another postcode letterbox cache, this time in the LGA of Coober Pedy. This cache was placed last year, but shortly after there was a significant rain event and so has had to wait for me to check that it survived. It did!
At the listed coordinates, you are standing in front of Coober Pedy LPO, possibly the only business in town that doesn't have a side hustle involving opals, although I don't know that for sure as I didn't go inside. Use the PO Box numbers to calculate the final location, a walkable distance away, although you will probably drive.
Let ABC be the number of the box to the left of #741
Let XYZ be the number of the box to the left of #943
The cache is located at S 29º 00.(B + Y)(A + C)(X - C) E134º 44.(A + B)(Y - B)(X - Y)
The checksum of minute decimals is 37.
I hope that like me, you find Coober Pedy to be a rather special place, and a welcome break on the Stuart Hwy as you head either north or south.