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

Released by Kyoto Microcomputer (KMC).

Images Provided by @GamingLegend64 (Twitter)

In the image below 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.

Shown in the image below, the heart of the UIC-MIDI is an Altera Cyclone-III FPGA.