Really if the identifier tag is gone so has it's identity.
Could be a Sandman, could be a Kingswood. You would be looking for signs of Blackouts around the sills, door frames, beaver etc.
Also should have black door handles (not pained over chrome). Console mounts in another identifier.

You could contact Holden Historic Services www.holdenhistoricalservices.com.au and if Ben can tell you it is a real Sandman then buy a certificate for future proof.

A lot of value has gone with the plate obviously.