Blackwing Eraser Refills are designed to be used with the iconic Blackwing ferrule and clip. 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.
Pack of 10 special-edition Vol. 57 Replacement Erasers. Each set comes with: 3 Red, 3 Blue, 3 Yellow, and 1 Grey eraser.
Blackwing Volume 57 available here.