portion of shortest radius substantially centered at centerpoint 24 as with the deltoid leading edge portions the leading edge portion tends to focus spoon engagement portion 28 at centerpoint 24 leading edge portion 23 of spoon mounting hole 22 in fig4 is cordate in shape that is shaped like the point end of the heart symbol the focusing of spoon engagement portion 28 in such cordate leading edge is apparent each deltoid leading edge portion described above forms an acute angle to serve as means to focus the engagement of clevis 25 in mounting hole 22 such acute angles are one highly preferred focusing means the obtuse angling shown in the embodiment of fig4 is also acceptable leading edge portions 23 of spoons 20 in fig1 2 4 7 and 10 each have a pair of edge portions 30 which converge to a corner at centerpoint 24 in contrast the mounting hole illustrated in fig3 has a curved leading edge without any corner formation each form serves to focus the engagement of spoon and clevis and provide the necessary center balancing which is missing from structures of the prior art mounting hole 22 in the spoon illustrated in fig5 includes a circular major portion 32 and a minor portion 34 projecting from major portion 32 in a direction toward front end 21 of spoon 20 leading edge portion 23 is along minor portion 34 of mounting hole 22 minor portion 24 captures and positions spoon engagement portion 28 of clevis inner edge 26 and tends to keep it focused at centerpoint 24 as illustrated in fig2 5 and 7 8b inner edge 26 of clevis 25 has a pair of edgepoints 36 in contact with leading edge portion 23 of mounting hole 22 the non circular irregularity of mounting.