The Atari Mystery Connection

I took apart the Atari 400 and noticed on the back of the main board chip that there is a connection not in use. What is this connection for and why is not in use?

Ki Jeong Yun

This connection, also found on the Atari 800, is used by Atari to test the machines on the assembly line. Due to its inaccessibility, it has never had any other use, although it could possibly be used for expansion.