As Yokozuna points out, that area on the reverse is opposite the largest/highest relief area of the obverse. I don't believe it's even a grease error. It's just a weak strike. The Memorial steps are also probably weak, especially near the bottom, because that's another point (opposite Abe's head) affected by relief during striking.
Tantum religio potuit suadere malorum.
"Just because you're paranoid don't mean they're not after you."
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