In the olden days the RAAF used used "civil style" call signs being VM-xxx* as distinct from civil regn of VH-yyy.
These were allocated by squadron and used for ATC purposes. EG. No.10 Sqn P-2's were VM-XD[x} where the last letter indicated the crew.
Tactical call-signs were used for exercises or training.
All stopped one day and squadron call-signs became the norm
*Nothing was painted on individual aircraft in that format.