This article also applies to the PIXMA MX439.
Make sure that the power is turned on, open the front cover (C), then open the paper output tray (D).
Open the paper output cover (E).
When the FINE cartridge holder moves to the replacement position, the head cover (F) OPENS AUTOMATICALLY.
*Document cover (scanner cover) is open in this photo. It is not necessary to open this to see the serial number