Well done! Based on the size in your hand, it appears to be a 2 reales. An 8 reales is the size of a silver dollar and very thick. The denomination is on the reverse, on the left side of the column about half way up the coin.
It's a little blurry, but it looks like 2 R
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Here is an undug example
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Great find for your yard. Hit is hard![]()