It took me a while till I found out that this "Vorratgeber", which is even for a German native speaker a strange word is nothing else than a fluid level transmitter with reserve indication. So it is not for hydraulics or engine oil, but it is most likely the fuel level transmitter. This means that this instrument was screwed into the fuel tank and measured the fuel level. See next image, which is an original WW2 description sheet I found.