The MCPX is home to the secret MCPX ROM.
Pin L21: PC Speaker
The MCPX has PC Speaker pin which can be controlled using the standard PC Speaker interface. However, no actual speaker is connected to the pin, so while the signal exists, there will be no audible sound on a stock Xbox.
The original Microsoft code does not drive the PC Speaker at all, so this otherwise unused pin can also be used for inaudible forms of unidirectional communication.