The mark consists of Circle within a circle, with the inner circle filled except for two parallel bars in the upper third of the inner circle, the two parallel bars having squared edges and a horizontal orientation, the upper bar slightly overhanging the lower bar on both sides, and four parallel bars in the lower half of the inner circle, the four parallel bars having rounded top edges and squared bottom edges and a vertical orientation, the four parallel bars having different lengths with the far leftmost bar being the second longest, the second leftmost bar being the shortest, the far rightmost bar being the longest, and the second rightmost bar being the second shortest.