Very much doubt the club can date a car assembled in South Africa either way and the old Rosslyn factory records will have gone up in smoke long ago.
Ask whomever you bought it from (assume the importer ?) to supply you with the original police clearance certificate issued when the car was exported.
You will be able to use that to register at DVLA in lieu of the missing SA registration document.
The 140z is a "south africa" only model , whereby Nissan ZA took a base B210 (120Y) and jazzed it up for local domestic market. I doubt that anyone outside south africa can help you with "dating" . These cars were locally assembled and so all supporting production records etc will be retained locally in SA in my opinion.
Can I ask why you need the date of manufacture? Is it for road tax free status ?
EDIT; just seen Chris's post above and yep, agree what he says. The police clearance certificate should assist, assuming there is one