I've spent a fair amount of time trying to find previous references to 50c coins like this, but nothing has popped up. I'm 100% certain that there have been others exactly like this posted in the Aussie error forum, at least 2, but possibly 3.
There was one CCF member who operated a lot of vending machines at the time and he said that 50c pieces often got stuck in the machines and that after he rescued them, they had these same markings.
Maybe someone else will have better luck than me searching the forum to find the prior articles.
life is a mystery to be lived not a problem to be solved