1973-D LMC What Would The Filled Top Of Bay 2 Be Called?

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Die subsidence (sunken die) errors are usually one-off events. In other words, the error will only affect one die. When die subsidence errors affect several dies, it's usually the periphery of the die face. This makes sense as differences in hardness would be expected to develop during flawed heat treatment (annealing, tempering, quenching). These processes cool or heat up a die from the outside in, and circular or semicircular zones of weakness can develop in the case of heat-treatment that is too rapid, too slow, or that is interrupted in some fashion.

The real mystery arises when you end up with highly localized, repetitive die subsidence errors in the same location well within the die face.

The most widespread epidemic of repeated interior die subsidence errors occurs among 1973-D cent. Only the reverse die is affected and at least 8 dies are represented. The defect is always found on the left side of the Lincoln Memorial, between columns 1 - 3. The outline and height of the zone of subsidence is slightly different from die to die, but the location is quite consistent. Why only the reverse die is affected, and why the abnormally soft patch of steel is restricted to this area, is a profound mystery. These defects are often mistaken for clash marks.http://www.error-ref.com/?s=die+subsidence

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I once collected the 1973-D in something like 14 different stages but freed them a few years ago thinking it was stages of a die clash

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As to value, they're not worth very much.