1788 Mexico 8 Reales Shipwreck Coin?

This is a 1788 Mexico 8 Reales coin that I had recently acquired in a coin collection. It sure looks unusual, and my first thought that it was dipped in some kind of acid. I was trying to authenticate it, but some things raised suspicion.
1. The edge of the coin seems to be chipping away, as if there is some sort of heavy plating.
2. The edge design appears to be poorly done or done wrong. I am no expert but this also could've been a "mint" error.
3. The only place on the coin that isn't unusually white, is the crescent moon shape on the obverse. If I place another coin of the same size on the white area, it matches perfectly.

-The coin passes the magnet slide test, and it makes the correct ping sound. Those small tests lead me to believe that is a solid silver coin and not plated.
-*The edge and design of the coin is eaten away on the edge opposite of where the supposed "other coin" rested on it. This could've been salt in the water eating away at the coin where it could've been underwater for a period of time.
-So, I have two questions. Is this coin legitimate? If so, do the clues show that this a shipwreck coin? Post any further ideas or opinions if you have any, and thanks in advance.