GameCube Missing Serial Port Cover
Early models of the GameCube included a mystery port labeled Serial Port 2. It's possible that this port was only used in development or maybe Nintendo had plans for it that they never followed on. Either way, Serial Port 2 was removed in later revisions but the bottom case still had a removable cover for it.
This is the same cover as the "Serial Port 2" cover, just without writing on it - blank.