Lovely piece you've got there.
It might not be as old as you think, production was probably sometime around 1955.
Here's what it says on the back:
http://www.radio-antik.de/images3/DSCI2600.JPGI took the liberty to translate that for you.
CURRENT SOURCE:
Built in battery: 4,5V 3LR12
External battery 6V or 60V. The latter is needed when the accuracy in the k range shall be +/-1%.
MEASURMENT:
Connect resistor to terminals.
If resistor is unknown set range switch to 500R. Press button.
If the needle swings to the right, turn knob to the left.
If the needle swings to the left, turn knob to the right, until 0-position is reached.
CHANGING THE BATTERY:
Open bottom by loosening the screw. Bend battery contacts like shown in the picture.
MAINTENANCE OF SWITCH:
Clean contacts with pure petroleum and lubricate them with a thin layer of Vaseline.