Vehicle Model Time.

[HK1837]

Got to say not a model I ever cared about, but I will take a stab.
Door frame window guide inserts, Chromed front bumper, Hub caps or grill, Paint colour, Front indicators differed.

No sheet metal changes only the holes drilled in them.

I agree with the HQ doors. Chrome stuff was part of the XY7 special vehicle package though. Paint colour yes but late HQ's were in HJ colours, HX got late HJ colours, some colours continued into HZ from HX plus there were fleet colours common for all.
HZ got the RTS badge on the guards unless RTS was deleted.
Later ones would have had black wipers.
Possibly the lion changed on th ehubcaps but hard to see from a distance.
Drum brakes would indicate 6cyl HQ or some HJ if you could see at the right angle.
You might sneak a look at a rear swaybar on a HZ.
Passenger side mirror may have been mandatory at some stage but some States had in mandatory anyway.
Clear front indicators on 1972 HQ then maybe in the right light see the parkers on and see the headlights illuminated on later ones.