What is the purpose of this machine?
It's what the developer Toaplan used to test JAMMA PCBs. Not exactly a supergun/control box type thing, but more like a passthrough with some test buttons, and what looks like a connection to a PC to view measurements or other data about the signals passing through, or to program it to send repeated inputs sort of like a macro. The PCB connects to the input and then the output I assume goes to a cab or control box.
EDIT: Considering the counters, I wonder if this was used for burn-in testing as arcade PCBs have to spend a huge amount of time up and running. Maybe it does some automated game playing or cycling through coinup->game over and the counters are showing the amount of times it's doing whatever they programmed it to do
EDIT2: The EXT/INT switch kinda confirms to me that this thing can be programmed to send inputs, with EXT being the inputs on the panel and INT for the sequence in its memory