There are two different kinds of ammeters used by the manufacturers and also available from the aftermarket. Gauges with shunts and gauges that require an external shunt. The '50 Chevy used a gauge with a internal shunt. My '67 has a gauge that requires a external shunt.
The first picture is a External shunt. The 2nd picture is of gauges with internal shunts. The 3rd picture shows that the 60Amp gauges have the shunt mounted outside, because they would get too hot to be mounted inside. the 20 Amp gauge does have the shunt inside as it will have much less heat. The 4th picture shows with the fat red arrow that 99% of the current flows through the shunt and less that 1% flows through the meter movement.
The 60 Amp Stewart Warner gauge shown, is considered a Internal Shunt gauge. SW has a separate Marine catalog and the Ammeters in that group require a separate External Shunt. My '67 uses a external shunt gauge, but they saved money by using part of the wire harness to serve TWO functions. The wire between the alternator and the battery is a measured length, with a specific resistance, that the ammeter sees as a shunt. The charging circuit, sees it as a charge wire connecting the Alt to the Bat.
