So it is a banana jack. I would be tempted to think it was a ground lead, except that I can't correlate that with the label "SWC".
Is "SWC" the whole label, or does that resistor cover up the last part?
It is certainly a battery-backup/inverter/automatic switch-over device of some kind. Both from the appearance, and what the company specializes in.
Unlike the general-purpose UPS things they sell for home computers, SEC have products for specialty applications like sump pumps and pellet heaters, etc.
Maybe that "SWC" bannana jack goes to some sort of sensor like water level in your basement, or run out of pellets for your heater?