Tony,
It appears there are two types of Ford ammeters used on the Ford CMPs then.
I took my information from the original harness after tracing out the wires. That round gauge ammeter has two terminals and is wired in series to the voltage regulator from the battery.
Second photo is the back of a NOS Ford Ammeter also with two terminals. The lower left gauge. The face of it is shown in a previous post and has the same face as the one you show.
Added a 1941-42 Truck wiring diagram showing the ammeter in series.
