To me (playing keyboard detective) the red flags are in the original purchase receipt
Sandy rego.jpg
The build plate says 12/76, the receipt dated 19/9/78 calls it a 78 model and says never registered before, meaning it was sold as a brand spanker. The typo on the year is neither here nor there but the price paid is $4,300 which seemed way too low for a sandman let alone one with a 308 and Turbo400. Thats less than a corolla van as per this highly reliable source http://nostalgiacentral.com/pop-cult...ces-australia/
If genuine, this should have been at least $7k and closer to $8k new https://www.redbook.com.au/cars/rese...b1970..1980%5d)) Also as per other car mags of the time I have but couldn't bother to dig out during Origin (Go Qld). The $4300 is the price listed for stamp duty so any possible trade would not be factored in.

I know there are better ways to test genuineness than this (eg;HHS as suggested) but that price seemed odd for what I knew