These coins are made of zinc, so expect them to have a heavy, dark patina.
Grading is one thing; more important is that is has no corrosion problems. If it has, zinc rot can be a major problem, and it should be quarantined from your collection, to stop other nearby coins being affected over a period of time.
Verdigris with copper and bronze coins can have the same 'knock on' effect.
This coin does not appear to have any localised corrosion damage, and so grading is possible, as with any other coin.
There is a tiny amount of wear around the eagle's beak and on the tops of the wings.
AU50 would seem to be reasonable for a grading.
Don't worry too much about the blotchiness, although it would have been better without this.
These coins circulated for a short period only, and so the whole issue did not see much circulation wear; the issue was replaced in 1949.
You have to be very lucky to find any pure zinc coin in pristine condition, that is more than 50 years old.
Nice find

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