The Korg Poly800 MDK (Midi Dump Kit) was an upgrade kit for the Poly800 mkI that Korg released for what seems like about 20 minutes in the late 80's. What is does is allow you to load and dump the entire patch memory to and from your synth and your computer via midi system exclusive (sysex) data. The original Poly800 mkI's had a very poor midi implementation, and only allowed you to save and load sounds from data cassette, like on early home computers  This upgrade saves you from a whole lot of messing around with audio data files and lets you swap and store sounds lot more easily.
The rackmount version of the Poly800 mkI, the EX800, already has the ability to transfer sounds over sysex, as the fact it has no keyboard and so therefore didn't need the keyboard scanning routines in the operating system gave Korg some extra space to add the facility for sysex transfer.

The build guide can be downloaded HERE,

This kit consists of an updated operating system in the form of the POLY MDK EPROM chip, a small MDK PCB, a couple of PCB mounts, and an eight way wiring loom. You will have to do a small amount of soldering, and also cut three circuit board tracks on the Poly800 main board, so you're going to need a steady hand.