This is a tester used to check the voltage of the dedicated battery case for the Nikon S motor drive. Insert the metal projection at the bottom of the tester into the socket of the battery case to measure the voltage. A resistor is built inside, allowing the voltage to be measured while connected to the motor drive.
On the bottom of the original box, there is a stamped marking that reads Battery Tester For Nikon Motor Drive and Made in Japan. The included gray cord is also the original from that period.
No signs of use and in excellent condition. The original box is also included. Perfect for your collection together with the Nikon motor drive.
