The convex portions 25 are formed as arranged in contact or substantially in contact with the points A1˜A7 of first group respectively that are provided on the upper side limit line 76 and the concave portions 26 are formed as arranged with a predetermined space provided away from the points a1˜a7 of first group respectively that are provided on the lower side limit line 77. On the other hand, the concave portions 26 are formed as arranged with a predetermined space provided away from the points of second group respectively that are provided on the upper side limit line 76 and the convex portions 25 are formed as arranged in contact or substantially in contact with the points b1˜b4 of second group respectively that are provided on the lower side limit line 77. Thus the upper and lower side limit lines 76, 77 that are arranged opposite to each other have the points A and a of first group and the points B and b of second group provided thereon respectively while the convex portions 25 and the concave portions 26 are formed as arranged in reversed relation in connection with the upper and lower side limit lines 76 and 77.