Canon CLC5151 Reference Manual page 368

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Finisher-W1/Saddle Finisher-W2/Puncher Unit-AG1 (Optional)
Insert the new staple cartridge.
NOTE
Only one front and rear staple cartridge can be inserted at a time.
Pull the stapler unit of the saddle stitcher unit towards you, and then
push it down into its original position.
Gently push the saddle stitcher unit back into its original position.