ball portion 258 260 each prong 266 268 270 272 274 276 278 280 of this particular embodiment includes a lug 286 288 290 292 294 296 298 300 respectively that protrudes toward the interior of the socket 262 264 formed by its associated prong the lugs 286 288 290 292 294 296 298 300 partially surround and thus rotatably lock a ball portion 258 260 within the socket as is clearly seen in fig1 the prongs 266 268 270 272 274 276 278 280 are spaced apart from one another in order to allow rotation and relative motion of coupled markers as the short pegs 334 336 are rotated into the spaces between adjacent prongs in the embodiment shown the ball and socket connection offers improved flexibility because the spaces between the prongs 266 268 270 272 274 276 278 280 allow the ball connecting pegs 334 336 to be rotated past the ball retaining lugs 286 288 290 292 294 296 298 400 of the sockets 262 264 as the marker is rotated in this embodiment the exterior of the socket 262 264 is square in cross section in other embodiments the exterior of the socket may have other cross sections or configurations according to the teachings of the invention the actual number of sockets disposed on each end plug 256 is not limited to two as.