"bears a mark or characteristic accepted as identifying and guaranteeing its fineness and quality"
Couldn't you argue that the denomination of the coin and the design itself is a characteristic mark? I accept it as identifying and guaranteeing its fineness and quality as much as if they stamped 1oz .999999 PURE SILVER, and surely the millions of buyers for the coin that there will be without any 1oz .999 silver stated on the coin, regardless of if they were to pay GST on it, are also proof that they are accepting this mark as a guarantee that it will be 1oz of silver.