From the designer:

I have a Panasonic MSX2+ FS-A1FX. This model does not have FM synthesis built in. I have 3 MSX cartridges:

• MegaFlashROM SCC SD
• GR8NET (mono version)
• Technobytes V9990 Powergraph Lite

The Panasonic, like most MSX computers only has 2 slots... But with some of the latest games like
Myths & Dragons released, I could easily use all 3 cartridges. The GR8NET provides FM music, the V9990 provides the graphics chip needed, and the MegaFlashROM is big enough to run the digital version of the game.

So somewhere around 2002, Hans Otten published a quick and dirty guide to building an IO Slot. Perfect for the V9990... Hans Otten - slot extender

With the ease of access to China to manufacture pcboards today, I turned to KiCad, and drew up an even simpler version. My version does not use ribbon cables or spliced wires and crimp connectors.. It is a pcboard, that simply adds the IO Slot. Their is no switch to disable the slot select on the primary slot.

NOTE: Unlike other slot expanders, in that it doesn't decode 'subslots', it simply adds a single IO Slot