David,
While they may not be exclusive to the carriers made in WA under one of the last contracts, I have noted that all the WA carriers seem to have this steel idler wheel and also (as is shown in your initial pic), an all steel return roller.
No doubt the authorities made refinements as they went along based on performance in the field and maybe in this case, the shortage of rubber - after Feb 42.
There were other changes made on WA carriers as well - one of them was a flange extension bolted onto the drive sprocket which was no doubt a way of improving track integrity, but possibly ended up causing a track fouling problem.
Jack
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