FDS expansion port thingy?

[S. P. Sour]

I have just gotten a Famicom Disk System, and today I noticed something - on the back of the RAM adapter, there's a little thing you can slide open to reveal a little slot, sort of shaped like the FDS end of the adapter but smaller, like / ------ \
What is it? Of the very little FDS documentation I can find on the Internet, there's no mention of it.
There are no words or labels near it.
Thanks in advance 

[maxellnormalbias]

I don't know what it was originally intended for, but you can use it to pirate FDS games with a cable from Tototek.

[P]

I don't think there is a cable that uses the expansion port, is there?

Anyway the FDS expansion port was never used by any commercial product as far as I know. It's common for game consoles (the NES expansion port wasn't used either).