I call it "the humpback", I have ran across them before in conjunction with the recurring die subsidence error. It corresponds with the area on the reverse. I started saving the 73D's that have this on the reverse to see how many different dies I can find.
I agree that the swelling along Lincoln's back is either an unusual form of Die Deterioration or a die deformation error. The distinction between the two can be difficult to discern at times.
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