The mark consists of a solid circular background containing four half obround shapes extending from an inner part of the solid circular background to the edge of the solid circular background, each quadrant of the solid circular background contains one of the half obround shapes, the circular end of each of the half obround shapes contains a circular hollow test port; the half obround shape in the above-left quadrant of the solid circular background extends outwardly at an above-left angle to the outer edge of the solid circular background; the half obround shape in the above-right quadrant of the solid circular background extends outwardly at an above-right angle to the outer edge of the solid circular background; the half a obround shape in the below-left quadrant of the solid circular background extends outwardly at a below-left angle to the outer edge of the solid circular background; and the half obround shape in the below-right quadrant of the solid circular background extends outwardly at a below-right angle to the outer edge of the solid circular background.