Mill balls are a thing, but I've not seen them in lead. That would be super soft to grind anything down. It could be a kind of grape shot for cannon use, or even just a weight to something.
I found all these in about a ten foot circle near a site that used to be a gun powder mill. I was thinking the same thing maybe some kind of mill ball. These ones are brass. Which would make sense in a powder mill you really don’t want sparks. Dime for size comparison.
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Is your merc a plain, S or D. The D mint mark is the holy grail of merc dimes