This is not a custom jig, but rather a modified one. :)
Boss Multiple Plugs (Modified)
A Boss J5 Multiple Jacks that was modified.
It now has 4 plugs instead of jacks.
It sends one signal to 4 places.
It was made almost 20 years ago to test 4 Track recorders.
(A cable was made to test 8 track recorders, but no pic was taken of that.)