Korg MEX8000 memory expander for various Korg synths from the mid 80s such as the Poly / EX800 and DW / EX 8000. Depending on the synth, this expander can allow up to 4 extra banks to be stored and accessed by the appropriate synth.
This MEX8000 has the battery cover missing and the back does not look cosmetically great. However, I used this a few months ago and it worked great. Not sure if I left sounds on it from my EX8000 or if I erased it.
Sensible offers may be considered but this is a rare and very useful piece of equipment so I'm tempted to hang on to it in case I buy another EX800 or 8000.