I searched for “Silver Bullet” in Exonumia and the rounds in the results are split: Many are under “Dollar (1785-date)” and others are under “Unspecified currency” (which is called “Unknown” when editing).
The Dollar currency isn't correct as the rounds don't have a dollar value. And Unknown isn't quite right either. It would be nice to have a currency definition of “no currency”-- “this item is known not to have a relationship with any formal currency”.
(There was a discussion a while ago where someone suggested “Silver”, “Copper”, “Gold” currencies for rounds. That would take case of the Silver Bullet items, but there are still other items in Exonumia where “no currency” would be better than “Unknown”.)
(I tried searching to see if this had been proposed before, but any keywords I tried produced lots of irrelevant hits. I'm sorry if I missed a relevant prior conversation.)