It's an expansion port, but there were no peripherals released that used it. If Sega had ever decided to release a "Master CD" CD-ROM add-on for the SMS, it would have used this port.
The Master System just treats it as another cartridge slot: if you look at the pinout, it's exactly the same as the main cart slot, but with a male edge connector instead of a female one. If you make a gender changer, you can connect standard SMS cartridges to it.
When you switch the SMS on, the BIOS checks each slot for the presence of a ROM. The slots are prioritised - it checks the card slot first, then the cart slot, then the expansion port. If it doesn't find a valid ROM in any of them, it runs the built-in game.
I personally found the expansion port useful when I was trying to dump Codemasters cartridges (for SMS Power). I wasn't able to dump them using the SMSReader, so I made a gender changer to connect the Codemasters cart to the expansion port (pic), then wrote a small program to transfer the ROM data from the expansion slot to a RAM cartridge in the cart slot. I could then dump the contents of this RAM cart on the SMSReader in the normal way.
OK, you need two 50-pin female edge connectors, like this (pin numbers shown):
then you just have to wire them together 1:1, in other words pin 1 goes to pin 1, pin 2 goes to pin 2, and so on.
If you're wondering which way round it goes, if you hold your cartridge with the label facing you, pin 1 is on the right.
The easiest way to wire them is to get two IDC (Insulation Displacement connectors) for 50-pin card edges, then just crimp them onto a 50-way ribbon cable. Suitable connectors can be bought from Farnell, cat. no. 635777. Make sure the cable is short, anything longer than a few inches may cause problems.
from what ive read that slot on the back was never intended to be used that it was just a cary over from the sg1000
the predecesor (i think) to the mastersystem a computer like system which was striped down and the mark III version was made ( mastersystem ) so the slot was just a caryover from the MB of the sg1000 and i think the case was too except the sg1000 was white