commenced the markings consist of a respective series of three rectangles 18 20 22 associated with each of the side holes 16 the rectangles 18 20 and 22 defined by the respective markings are of similar shape to each other but are arranged in decreasing size the smallest rectangle rectangle 18 of each series of rectangles is positioned with the respective side hole at its centre and with one side of the rectangle 18 replaced by a portion of the perimeter wall 14 the intermediate rectangle rectangle 20 and the largest rectangle rectangle 22 of each set also have one respective side replaced by corresponding portions of the perimeter wall 14 the large rectangle 22 and the intermediate rectangle 20 of each set are arranged symetrically with respect to the small rectangle 18 of that set additional markings 24 are provided on the base and these markings 24 interconnect the two corners of either end of each small rectangle 18 with the closest pair of corners of the other small rectangles 18 thus four trapezia 26 are defined which together with the small rectangles 18 define an octagon further markings 27 which are parallel to one pair of sides of the base 12 define a further rectangle 28 the rectangle 28 has two.