From the manufacturer:
FISKARS-Circle Cutter Refill Blade Carriages. These refill blade carriages are designed to work with the Fiskars Circle Cutter 9380 only. They slip into the cutting arm easily and cut various kinds of paper, cardstock, photos, and other thin materials. This package contains two refill blade carriages and directions for replacement on the back of the package. Caution: Blades are extremely sharp. Handle with care and keep out of reach of children. Imported.