I was looking at this one earlier. Maybe overstruck? The patina is thick. I am not sure. The toning looks like it is flaking off. That usually happens with white metals not Silver.
I probably can't help, I did find this comment on CT from years ago (about another coin) that mentions plated fakes. Have you weighed it?
I don't have the link thing figured out here, sorry. But if you Google "cointalk cap & rays plated" it should come up. But I believe that coin was decided to be ok.
All you have to do is copy the url and paste it in the your message here: https://www.cointalk.com/threads/help-1835-cap-rays-8-reales.301136/