Well, FWIW, the coin was cast from a salvage piece, and a very decent chance it was of a Maravillas piece. Most salvaged Potosi 8R of these years are from the Capitana and Maravillas wrecks.
Note, however, the flip insert says Mexico whereas this is a repro of Bolivia... though ppl are sloppy with that commonly.
Aside from the highlighted portion, the filled in crack, some prominent casting zits... and to the trained eye, the overall "mushy" look to the surfaces beyond what seawear does.
Not a repro I've seen before, which is curious since it's apparently older.

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I don't know how you prove it's a ship wreck coin certainly a hand written note is not proof
Obviously without a photocert, it is sort of just a coin with a piece of paper. However, if you know what types of coins were usually found on certain wrecks... and additionally, what they typically look like after "conservation" (though different batches of coins from the same wreck may have been cleaned differently or just show vastly different levels of preservation in general)... it lends some assurance.