The mark consists of substantially of a contoured base plate having a substantially horizontal center section with a first and second end with a first integral inclined plane at the first end of the center section and a second integral inclined plane at the second end of the center section, the inclined planes being at equal obtuse angles to the center section with upper surfaces of the center section and inclined planes being in alignment, the center section including a circular collar centrally positioned on the lower surface, the circular collar having a concentric, depending, externally threaded collar, the contoured base including an elongated second layer affixed to the upper surface of and conforming to the shape of the center section and two integral inclined planes, the second layer having a greater length and width than the contoured base and extending laterally outside the plane of the contoured base with ends of the second layer being folded over the outer ends of the inclined planes and abutting and affixed to the underside of the tips of the inclined planes. The broken lines are intended to show the position of the mark and are not claimed as a feature of the mark.