Here's the coin in question:
First, why would someone buy silver coins and then try to deposit them at the bank?
And second, why wouldn't the bank take them? Legal tender is legal tender as far as I know.
So, one must ask, what does it mean for a national mint to mark a coin as "20 DOLLARS", yet not have it be interchangeable with other currency that is also labeled as 20 dollars? If it's really a numi, why label it with a dollar value? This is another example of why the world's fiat "money system" is at risk. It's already bad enough that money can be established by decree, but now even the decree is open for interpretation.