Rostock is a delta robot 3D printer.
This part is the vertical carriage for LM8UU and universal joints to attach two diagonal rods.
Newer versions of the OpenSCAD source files will be published here: https://github.com/jcrocholl/rostock
Print three of these and attach each to a pair of LM8UU linear bearings with zip ties. To improve the horizontal constraint, you could use LM8LUU (45 mm long) instead of LM8UU (24 mm long) on one side, but this is optional.
Screw a M3x16 with a round head into the vertical hole to adjust/calibrate the top endstop. You shouldn't need to pre-cut the thread, it should fit very snugly if your printer is calibrated properly.
Put four M3 nuts inside the universal joints. Then insert four M3x12 screws with washers through these nuts and into the universal joint pieces. Tighten them pretty well to prevent the delta robot from unscrewing itself.
For all fasteners I recommend stainless steel.