I have an old furnace that has square d circuit breakers integrated into the system. I have to replace one. I ordered what a thought was the correct breaker, but it turns out it has a small hole on the back side. a small spring is visible inside. the hole looks as if it accommodates a wire or connector. I have no idea what it is supposed to do. can anyone say what the hole is for and whether the breaker will work as a replacement? Now that i've seen it, i see that all the breakers online have the little hole. the breakers are not like the ones that go in service panels. each one has wires in and wires out. thanks.