1931 Series 43 Instrument Wiring: Capacitor

On my ’31, there is a black capacitor, about 2 3/ inch tall x 1 3/8 inch diameter, with the wire connected to the passenger side, black wire terminal of the Ammeter. It is not shown on the schematic from the “Wiring And Tune-Up Guide” so does anyone have any idea why it is there? This car does not have a radio, but I have no idea if it ever had one.