The description of the green car is a stretch in my opinion. There is no evidence that it was one of the works cars, and actually quite a bit of evidence to suggest that it cannot be a works car, as it was still in South Africa as the two works cars were arriving in New Zealand from the US. And that's before we get into the thorny issue of how the sports car that was built from it can exist at the same time as the Cooper.
I would see it as a resurrection of an ex-Brabham Cooper T53, but to try to assign a chassis number to it is overambitious. I have said "maybe" F2-17-60 on my page.
http://www.oldracingcars.com/cooper/t53/