devillesadvocate Based strictly on appearance, I would say the odds are good that the coin is genuine and silver. The numismatic value is extremely limited unless there is a story behind the initials.
To confirm authenticity, Specific Gravity would be your best bet as long as the coin can be removed from the chain.
The coins of Potosi were notoriously variable in actual alloy which is why many countries did not trust them at full value. It is why the Chinese never paid a premium for these coins unlike those made at Mexico City.
This coin would actually be worth significantly more if it was a counterfeit made while these coins remained in circulation.
My book on Counterfeit Portrait 8Rs is available from Amazon http://ccfgo.com/TheUnrealReales
or from me directly if you want it signed.