Interesting!
8443 is the GM code for a C60L, yet your vehicle appears to have a 134in wb making it a C60S. Is there any sign the chassis has been cut and shortened?
Contract E52-384 only
appears to be for Ford F60S and F60L trucks (so far!) for the RAAF. R100 is a GM-H body code for an RAAF vehicle and the M in the serial no indicates Melbourne assembly. Other RAAF Chev/GM-H vehicles have appeared under contract marked SM-E52-448