Many factory "ammeters" were actually milli-voltmeters with zero pointing straight up. You can tell this type of meter by the small gauge wires going to it. Usually one lead is connected at the alternator (or voltage regulator) and the other lead is connected at (or near) the battery. When the voltage at the alternator is a little bit higher than the battery the gauge reads "charge" (positive millivolts); when the voltage at the alternator is a little bit lower than the alternator the gauge reads "discharge" (negative millivolts). On the systems I've messed with both wires were fused; if either fuse blows the meter doesn't work, but most people don't even notice. I've never seen one with numbers (that doesn't mean they don't exist); they usually just have "D" for Discharge and "C" for Charge on them.
Most aftermarket ammeters are the real-deal, true ammeters that have to be wired between the alternator and the battery in order to give a meaningful reading. This results in some really big wire. It also means that when the battery is charging or discharging, the entire load is carried through the firewall (probably twice) in order to get through the meter. Lots of times they get wired up without any fuses, meaning there is an unprotected wire (or two) going through the firewall. That's a really good opportunity for a fire.
I don't think ammeters produce very much useful information. In my opinion, a voltmeter is much more useful and tells you a lot more about how the system is functioning.
