Ahh, good to know what sort of car. So like quite a few NZ assembled cars in the 1970s by Todd Motors where shells were shipped here with no chassis numbers stamped and not necessarily ever were (ala Datsun 1600s and some Mazdas) and they just relied on a manufacturer ID tag.
So the ID tag on the car, on face of it, is for the wrong body style and recorded manufacturer (Chrysler/Hillman) and that is what leads you to believe it has had the plates tags swapped? Has the question been asked directly of the current/previous owner?
It may be just a mistake. I have seen a ton of mistakes for old cars. Peoples recording ability in the 70s-80s was just absolute rubbish. If it was assembled here in NZ then it could have been some muppet on the production line that put the wrong tag on the wrong shell. You could investigate this further. If you contact Land Transport they should be able to tell you any history of colour changes to the car.
ie, its original details may have been changed - colour or body style. It might be obvious that the car in front of you was originally brown, or you have scratched some paint off and seen brown at the bottom (unlikely someone has EVER done a bare metal interior/exterior paint on an Avenger.) If the original colour for the plates, matches the car in front of you, this could indicate a mistaken ID tag put on the car.
I have used the above before. A car I have owned had a chassis number stamped into it that looked like it had been done by a kid (wonky) and in a different place from JDM assembled models. But after confirming chassis number range allocation for exact model year for NZ assembled cars as well as the different chassis number location directly with the manufacturer I took a further step. I confirmed with Land Transport that the car was originally registered as yellow and had been changed to green in 2006 and blue in 2008. I could see areas of the original colour in the boot were yellow under blue and green. I was then satisfied convinced that the car was legit and the person who stamped the chassis number needed their eyes fixed.
A further glimmer of hope is in the engine no. Land Transport (not car jam) can search via engine number. The details will only be lost if the original car was deregistered around 1990.
Failing that all you can do is ask a Land Transport inspector, spell it all out and hope you get one that is up with the play / cares about old cars. Even that is difficult.