Sounds like a pretty standard CHT/EGT instrument. The top of the bar is likely EGT, and the "missing" bar (blank bar) indicates the CHT (which is always cooler than EGT, it's a way to have both temperatures read on one graph). The mystery button is likely a leanfinder, it will make each cylinder's EGT flash when the temperature starts decreasing (IE, peak EGT). You'd press it, slowly lean the mixture until the first bar starts flashing, then go back rich a certain amount of degrees (usually 50-70) for best power. Or, press it, lean until the last bar flashes, then keep going lean until you are a certain amount of degrees (usually 30-50) lean of peak for best economy. PM me if you have any other questions.