Model E-36, this Electric cooktop looks to be about 20 yrs old. Two of the 4 burners has a depressable button in the center that depresses when a pot is on the burner. Those 2 burners also have infinite thermostat settings where the other two without buttons have 6 temp settings from low to high. Does anyone know what the burner buttons are for and why the heat controls are different.I assume that these are low heat burners with greater precision.