I have seen this on steel cents before and I have always assumed it was caused by contaminated metal stock(planchets were punched after plating the stock) that was not properly cleaned before the plating process which eventually caused subsurface corrosion in a similar manner to Zincolns. For the reverse, it is rather uncommon to have a random die chip in the fields unattached to any device so it is possible that it is a die dent.
That's a really nice steelie. Probably close to 65 on the reverse as would the obverse without the little raised dots. I agree with an assessment of PMD in the form of a chemical reaction to some form of atmoshperic exposure. Also, I have seen this as well on a number of steel cents.
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