some possibilities:
the chassis chev 125 = 125" or long box? you can measure that
max GVW 2tons would be a 1/2T, and maybe the 12CWT is Holden saying it'll carry 12 CWT, which may mean a 3/4T
I don't have a list of the AD series CDN VIN codes, but someone else here may
body trim and paint will be hard to find the codes for
top acc 12??
the 1/2T would have a single pack of 8 leaves on each side - heavy 3/4T would have 2 packs each side, 8+5 leaves - maybe the rear has been changed, or Holden did something different?
can't help more, but there's some Aussie truckers
on the board who might know more about it Bill