The 1/2 Real has the stars that are on the obverse. Without knowing size or weight I would guess that it is a 1/2 real. http://www.ngccoin.com/price-guide/world/peru-12-real-km-51-1751limae-1760-cuid-2051-duid-9873
It's not an 8R as the edge scalloping appear to better fit a smaller coin. The two "star like" things next to the shield on the obverse are "rosettes" and denote it as a 1/2 real. The M with the circle above, denotes the mint as Mexico City for the ViceRoyalty of New Spain. Unless there are some edge bangs I can't see, I'd call it a nice VF. The silver purity is 11 Decimos or 11/12th pure or .9166 fine. 1758 is a common date. Dan Sedwick (The cob & Pillar specialist) is selling a VF one for $85 but yours looks a tad nicer.
