Chevrolet MCP 3 Ton Lorries G.S. (Australian)

[motto]

If only it were that simple Tony but unfortunately it isn't.
TM 10-1677, the maintenance manual I mentioned earlier, was published by General Motors Overseas Operations and is identified as being applicable to :-
CHEVROLET 1-1/2 TON (Right Drive) 4x2 TRUCK
This is the same manual that has the strip of paper on the cover that gives the payload as being over three ton. (GVM minus Tare) And also lists why, frame reinforcement, aux springs and tyre size. The frame reinforcement can be readily seen in both the photos you posted and tyres appear to be similar on both trucks.
You usually can't tell whether a vehicle has a two speed axle from a photograph and a brake booster could be inside the chassis but usually found on the outside. What can often be seen is whether the front axle is the light duty with bicycle type cup and cone wheel bearings (most of them) or the heavy duty type with barrel rollers. One of the things that has puzzled me over the years is that, by direct observation, the installation of light or heavy front axle appears to be completely indiscriminate and unrelated to differential or booster installation. There was a class of truck produced that was identical to the '42 vehicles being discussed but it was shorter with a nine foot instead of a twelve foot tray. These trucks were classed as being 30 CWT but all seem to have come with the heavy front axle. This was confirmed by a fellow that used to work for Regent Motors, a GM dealership. It just doesn't make sense.
Brake booster wise, I've only ever seen the diaphragm type PBR boosters on these trucks which I have always taken to be a post war or post service modification. The North Americans seem to have used piston type boosters on their trucks which is what I would expect to see if original equipment.
In summary, these are interesting vehicles produced at a time of duress and shortages using pretty much whatever was available with a lot of what went on not being very well documented. We all know parts of the story but are often left guessing.

David