Korg Mono/Poly

Hi, I'm a new member as of a few minutes ago!

I hope you all don't mind, but I made a new photo album called "mods"
for people to share their Mono/Poly mods, and uploaded the first photo.

That said, I got mine used with this mod already done, but I have no
idea what it is for! Can anyone please have a look and get reply to
this post? I've scoured the internet for an answer with no avail. The
closest thing I could find was some Moog hook-up port thingy, but that
could be totally wrong...

Thanks,
Casey