Nope, I've seen that extension too, in HNOstar magazine #43 if I remember correctly, and it's just an I/O slot. Very handy for GFX9000/Moonsound/Music Module (who needs the built-in program anyway? ) and the like.
No this is a full working slot. I have already connected: FM-pac, Konami SCC and normal cartridges and Sunrise CF reader to it. And all works.
This slot uses slot 3.1
What do you want to see? I have my computer open with this mod, can make pictures tomorrow. Just let me know what you want to see. Not that it is that exciting to see the hdd male connector on the mainboard