Buffalo Nickel surface damage?

What do you think caused the surface damage to this coin. Just curious as it looks like some kind of acid treatment.
Jim
When a man who is honestly mistaken hears the truth, he will either quit being mistaken or cease to be honest....Abraham Lincoln
Patriotism is supporting your country all the time, and your government when it deserves it.....Mark Twain
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Comments
Kinda looks like a grainy planchet that didn’t fill out when struck would be my guess, Jim.
Looks like a bad strike on a Type I Planchet. The rim looks weird, too. Is it a D or S? I'm pretty sure it is.
Could also be a light strike created when the press was shutting down. I've seen other Buffs with a flat strike like yours, with the entire center area flat and featureless.
Pete
Both! OPSI marks in the center of weakness, corrosion at the top. Great teaching coin!