embodiment of the invention the transitional surface 54 is inclined obliquely relative to the central axis a 1 of the shank 16 and faces generally longitudinally toward the leading end of the receiving portion 20 the transitional surface 54 accommodates for the difference between the shank guide and locating diameters d 1 and d 2 shown in fig4 the locating portion 30 terminates at a second radially extending surface located at the leading end of the locating portion 30 the second radially extending surface may be defined by a pair of radially extending surfaces 56 which extend perpendicularly to the central axis a 1 of the shank 16 as shown in fig5 9 13 and 15 or radially extending surfaces 58 which are inclined obliquely relative to the central axis a 1 and face generally longitudinally toward the leading end of the shank 16 as shown in fig1 the second radially extending surface may function as an axial stop surface in the place of the first radially extending surface described above the shank drive keys 32 are located at the leading end of the shank locating portion 30 in a preferred embodiment of the invention the shank drive keys 32 have an axial profile dimension d 3 shown in fig5 and a radial profile dimension d 4 shown in fig4 the radial profile dimension d 4 is preferably smaller than the axial profile dimension d 3 to enable the shank drive keys 32 to be closely aligned with the central axis a 1.