This is a high-grade (AU?) example of an 1842-dated, Zacatecas Mint, OM assayer initials contemporary counterfeit 8 reales. My guess is this example was likely struck on a German silver planchet. This variety is currently documented as a 'singleton' (and not part of an established family), however I strongly believe this will be attributed to a larger group of counterfeit varieties one day.
 
The attribution in the title of this listing is my own personal attribution code for these counterfeit 8R varieties in association with hopefully a forthcoming publication (one day).