Don't get overly paranoid. As Sap said, what are the weight and dimensions? If the weight and dimensions are good, and the bar has no obvious signs of tampering, then it's fine. It's supposed to be 19x19x5mm. An ounce of gold is very small and very dense. The tungsten worry is almost too remote to even consider. Just check the above and put your mind at ease.
It might have environmental damage and if it doesn't then AU+ sounds fair. I'd try soaking it in pure acetone for a few hours. There is definitely something on the surfaces and it needs conserving. But don't clean it.
64 looks conservative here. A factor is the coin shows no signs of dipping that is prevalent on Morgans. The virgin surfaces alone are worth a full point to many collectors. 64 is fine as a technical grade. A market grade of 65 is deserving.