Measure voltage of the panel light bulb inside a vintage radio
Perhaps (probably) the lamp is being run on AC. Try switching your multi-meter to an AC volts range. You will not see any voltage indicated on an AC range from the battery (on most low-end meters) but it should show the lamp voltage. 

6.3VAC would be a common voltage for a North American designed product, not sure about Japanese radios of that vintage. Depending on the type of bulb the operating voltage with the lamp connected may be considerably less than the lamp rated voltage, to get more than the lamp rated life. 

The once-ubiquitous #47 bulb had a life at full voltage of 3,000 hours or just a few months 24/7.