G'day AUSMV,
Well done on picking up a great looking ambulance, it looks very complete.
It would be interesting to know which airfield it served on, Oakey would be a possibility, as a field run solely by the Army.
The light signals decal is, as others have said, a reminder of the possible signals the tower can use to control vehicles around the airfield.
The tower also uses them to control aircraft with radio failure or no radio ops.
The controllers have a 3 colour light that usually looks like a big spotlight for this job.
Most vehicles were / are fitted with a radio, so the light signals are only a backup if the radio fails.
You do not usually want to see the last one, a visit to the tower after that one is unlikely to be just social!
Cheers,
Mike.