This is for a modified version of the Darwin's Z-axis. You can put it on 3 threaded rods instead of four to reduce the risk of over constraining the build platform (that you have with four). This is for the same reason why a tripod is always stable, but a four legged table will wobble on un-even surfaces unless the legs are adjusted.

I'm hoping that the Z-axis will run more smoothly (less resistance from running it through the M8 nuts and less inertia (1 rod less). You need two of these parts and two skate bearings to fit inside it.