The gutter will be copy the shape of what it was struck through. Round would be a wire/bristle issue. A flat bottom would be a cut with a knife. Also on a knife cut the metal would be raised on the areas of the field next to the cut.

A deeper cut will cut through the plating. A mild cut will leave the plating in tact. So if you are not seeing any gray, it maybe a struck through error on this coin. (The obverse doesn't show a push out) So I feel this is a struck through wire/bristle issue. Too thin/width to be a rim bur.
CoopHome:
What will a struck through wire of cleaning bristle look like on a zinc coin? the affected are will show no zinc on the plated/incuse area. the opposite side will not show a push out. the strike though will make the incuse mark, but the dies prevent any rise on the opposite area of the coin.