Something dragged itself across the die face, and in doing so deposited some crud on the surface of the die. The crud was deposited only on the field. Imagine a paintbrush being lightly stroked across a die face. Paint will only be left on the field portion of the die. In this case the "paint" was more like a mixture of grease and grit. I doubt it was left by a dirty feeder finger, as the direction is wrong.
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