This is an enclosure for the MidiThru4 by DeftAudio 1x4 MIDI Thru PCB.

The bottom contains two bosses that help the PCB snap in place. It also contains debosses legends for which MIDI jack is IN and which are OUT.  Labeled holes for POWER and ACTIVITY are also present.  The top and sides contain a debossed logo to remind you what's inside. 

The Fusion360 file is included if you want to modify to taste.