The early starter buttons are all chrome, have one wire to them and provide a ground path to the starter solenoid. I am more familiar with the later pickups, but think they are all similar. On the later ones they change over to a solenoid that received power from the starter button. The center button is normally black plastic on them and they have two wires. One connects to a power source and the other one to the starter solenoid.
If there is any question as to type of solenoid it is, it is easy to test. But, be careful!!! Be sure the vehicle is in neutral and the wheels are blocked from moving. Take a short jumper wire and jump the center terminal to a good ground. The starter should engage. That would be the solenoid for the early grounded starter button. If nothing happens, try shorting the center terminal to the Battery terminal of the solenoid. If that engages the starter, then it needs the later power style starter button.