This is the filament-pulleys or hobbed pulleys on all Original Prusa MK3/S and MK2.5/S. The set includes two gears (one with a set screw and one without screws), two bearings, and a shaft. The gear with the set screw has to be firmly tightened over the flat part of the extruder motor shaft. The gear with no set screw must be installed to the idler door, where the bearings and shaft allow it to rotate freely.
