Descent guess, but you are off by a hundred years. It is a Potosi 1 Reale minted in 1764. They were the last colonial mint still producing Pillars/Waves cob coinage where the others had switched to milled coinage and Pillar dollars. Consequently, the quality they produced was absolute crap. I don't think they cared at the time figuring there was no oversight. For collectors, it makes finding a descent specimen difficult for the late Potosi period. The assayers (yes, plural) was Jose de Vargas y Flor and Raimundo de Yturriaga.