Battery voltage identification


Pinball Nudger
Brand new to forum and am seeking some help . Working on machine from the mid 30's . it has 4 batteries ( dry cell ) . Rayovac dry cell class A number 6 ignition dry battery for radio and all general uses . No voltage is indicated on battery .Machine has a small black motor which I opened and cleaned . Put the motor on a 6 volt charger and would not run , flipped the switch to 12 volts and the motor runs .if these old batteries are 6 volt each that would be 24 volts ? if they are 1 1/2 volt then the charger should have ran the motor at the 6 volt charge , right ? It is a very small trickle charger and is putting out around 9 volts on the 6 volt charge . Pinball machine is a Jennings cocktail hour payout .
If anyone has any advice please feel free to pass it along to me .