The mark consists of a three dimensional, visible configuration of a clover-leaflet shaped, upwardly facing, stand-on zone bordered by a relatively thin rim defined by a pair of substantially straight side surfaces that angle toward each other as they extend rearwardly from a curved front surface of the rim which joins smoothly with the two side surfaces of the rim, and having three elongate, rounded-end slots that open upwardly through the stand-on surface including two straight-sided elongate slots of substantially equal length and width each spaced inwardly from and extending substantially parallel along a different one of the side surfaces, and a curved elongate front slot spaced inwardly from and extending along at a substantially constant distance from the front surface.