That is just a conventional pattern ambulance body on a 1941 Chev chassis and probably built by General Motors/Holden. The RAAF used a lot of these while the army had the 'Indian Pattern' body on a variety of Chassis including Dodge Ford & Chev, although the Chev was more common. The Army also had CMP Ambulances in the 'Australian style' which is similar to the Indian but not attached to the Cab and then The CMP F30 type which were larger versions of the Indian style on a Ford CMP chassis but with double rear doors rather then canvas..
You also then had the Canadian style ambulances with double rear doors and then the British Austin K7's, the American lend lease vehicles etc. and of course the multitude of other Medical type trucks like Blood banks, dental etc.
This still does not answer my original Question so I am beginning to think that no one knows hmmmmmmmmmmm.
Cheers
Cliff
Pic 1 - Australian style Ambulance