Blackwing Pencils have always been known for their iconic, replaceable, square erasers. This is a set of 10 little Replacement Erasers, so you never have to retire a pencil too early.
To replace your eraser, slide the eraser and clip out of the ferrule, remove the clip from the old eraser, place the new eraser into the clip, and insert the new eraser and clip into the ferrule.