This nifty little device is the PARTNER-CTR “UIC-MIDI” for 3DS music and audio Development.

Released by Kyoto Microcomputer (KMC).

In the image above you can see the USB connector and standard DIN-5 MIDI port for connecting external Midi devices like Keyboards or midi sequencers etc. The USB port is also used for performing Firmware updates and other development software related control. The heart of the UIC-MIDI is an Altera Cyclone-III FPGA.

Images Provided by @GamingLegend64 (Twitter)