Holden were years behind the ford boys in documenting their classic and muscle cars.... Fords are documented back to XR / XT Gt.... The HDT didnt surface until 1980.... So unfortunatley there is no chance of getting a sandman authentication certificate....

The best thing you can do is open a thread on here documenting with pictures all the key sandman identifiers of your car.... If its the real deal the experts will tell you quickly enough.... And that will be your authentication....

Good luck with it..