Welcome to the forum.
We would probably have to know who built the dual cab/ cab extension. Maybe it was a "factory" dual cab from new. I do not really know but I am guessing that the factory bodies were made by Chrysler in Adelaide?
Dodge or International people might know, as Inter and Dodge shared the sheetmetal.
You will probably get some better information on here in the next day or so.
I have my shoulder to the wheel,
my nose to the grindstone,
I've put my best foot forward,
I've put my back into it,
I'm gritting my teeth,