genesis serial port ?

I've got 2 genesis (hugh 2 megadrive called in France) and one have got a strang port on the bottom. for what is it ? Maybe a modem ?
no, the port is near the a/V out. it is called "ext"

the mega cd port is on the right not on the bottom !

the model is the following 1600-09. i've got another one genesis a 1601-09 and the strange port have just disappear !
that's a serial port that was used for the japanese modem.. or supposed to anyways, it never had much success, so it was removed.
i'm going to sleep not silly as I was in the morning.

thanx guy

p: a modem on a genesis what a strange idea !
X-Band was its name and I miss it and would LOVE to see a WORKING US version emulated....even if the modem didnt work...I want to see the old interface again....