A list...

[thatguy96]

Sounds like the really early versions of the US 75mm airborne howitzer series. IIRC there were three, the M1, the M2, and the M8. I don't have the relevant book on me to know for sure.

[TDR]

Unit #145 USA Army is similar to Unit #260 UK OOB.
The pic in the US OOB is better.

The difference to the one I have seen in the Aust pics is that the wheel base/axel length seems shorter an the wheels are not the same as in the 2 OOBs, but as I said wagon style, with spokes and thin tyre section.

There were some pics of 75mm guns with normal rubber tyres but they were not easily seen and could have been standard 75 mm guns or with US troops on a joint trg exercise.

Though the pics indicate that the guns were there then it does not indicate when they were issued. Since the early pic is 4/44 and in PNG the start date will possible be late 43 or very early 44. At present I would look at a start date of 1/44 till something more concrete comes along.