Lionel,
IIRC, the Inters (AA/AB, etc.) use the number as an indicator of the GVM in pounds. For example, an AB160 has a gross of 16000 pounds (7,257 kg), the AB120 is 12000 pounds (5443 kg). Our car licence and rego has a break point at 4500 kgs, so you will need an LR/MR licence to drive them and also have annual machinery inspections and commercial rego.
I am also interested in an old truck, but like you, it must have a GVM of 4500 kg (just under 10,000 pounds) or less OR be capable of being derated.
HTH