Forgive me, in advance, if the answer to this question is obvious. I am new to the Apple II series of computers.
I recently acquired an Apple IIGS, on Ebay. After reading various manuals on the web, I have learned the functions of most of the ports. However, the function of one particular port eludes me: a rectangular hole on the far right of the computer (labeled "13" in the following picture).
Could someone enlighten me as to the function of this port? I have found no information about it.
Thank you, in advance, for the assistance.