CraigB/Charlie,
While only one ammeter is listed in the '32 chassis parts catalogue, there were three '32 model generators that had outputs over or near 20 amps and this ammeter was perhaps intended to be used with one of those generators (BB-10000-B at 22 to 24 amps used in four-cylinder busses, 18-10000-H at 17 to 19 amps used in V-8 police cars, and 18-10000-M at 22 to 24 amps used in V-8 busses). All other '32 generators had normal outputs of 10 to 12 amps.
The non-police radio installation instructions for 1932 models make no mention of a unique ammeter (or generator).
A painted finish was substituted for chrome or nickel plating on service part components produced during WWII, which explains Jim's ammeter's painted bezel.