By far the oldest coin I've ever found. It was in my yard, down about 6 inches. Best I can tell it's a 8 reales. 1775 is clearly the date. And can read Carolus III.
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.