Could I see a closer shot of the area in question? Sometimes you find grease that is clear on the gutter area of coins. A toothpick can let you know if it is remove it. I've seem pieces of plating struck through on Zincolns. This would be called a lamination. But what color that area. It appears brown. If a layer of plating was missing in that area, it should be gray. So there are a lot of variables to consider. It could be glue or even something else. So a closer image might give us more information.