Race cars and bikes that competed on closed public roads had to be registered. There was a category called "Class A" that covered vehicles and machinery that were used infrequently on public roads eg tractors and other farm machinery that had to travel on roads to get from one farm to another, trucks that were used seasonally to cart fruit etc.
Racing cars and bikes were Class A, sub category E. I still have a couple of old stickers here and would post photos if I knew how.
The cost was $1-30 in 1973-$1 for the 2 metal plates and 30 cents for the actual rego.
The rules around using race cars on the road were very loose-there was a clause that said a vehicle registered EA could be driven on public roads "for the purposes of testing or servicing". The boundaries seemed to have been pushed at times-Jim Birkett crashed the JBS now owned by Garry Simkin on the Ngauranga Gorge on his way to or from a hillclimb and I have photos of the Cooper I owned parked on the side of what is now the Hutt Motorway by the original owner Bob Gibbons.
As late as about 15 years ago, Colin Waite and Allan Woolf were the only ones who could run an out-and-out race car at a HB Car Club hillclimb at Pukeora because the Cooper Imp was registered EA and the organisers had obtained the wrong kind of road closure. Further, the little Cooper had a "daylight Warrant Of Fitness", another quaint anachronism that probably does not exist any more. Not only was Allan able to do the hillclimb, after each run he drove the car on State Highway 5 back to the start line! I always kick myself that I did not get a photo but on one trip back to the start line there was a big stock truck and trailer closely following Allan. Allan gave a perfect hand signal and turned right to get back into the venue. The diminutive Cooper looked positively miniscule in front of the huge truck.