I would guess paint or stain. Notice the rim, Lincoln's cheek, part of the date, and some of the letters were some of the paint/stain has been worn away.
Years ago they tried using varnish or clear lacquer to protect BU coins. It ruined the value as the surface was altered as a BU coin. Sometimes clear nail polish is added to a coin. When this wears off, it looks like the coin posted above.
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