Please need advice before buying this Dos Mundos, genuine or fake?

hi there
I am new member here , I am beginner in collecting cobs escpecially those Dos Mundos so I have to learn more & more and need your advices to tell me if this 8 reales 1770 Dos Mundos is genuine or fake , material is silver and weight is correct but as you know it's not sufficient to judge ; sorry This picture isn't good quality but it's the unique one I have to show you
the coin is proposed to me by a particular who swears that's genuine & proudly says he's professional & never has been fooled by anyone when buying coins especially this 8 reales.
thanks more