Mounted on top of each JU 52 engine is a translucent tube and a dial (see images).
What is the purpose of the tube and what does it measure?
Does the dial measure (oil or fuel) flow or quantity?
Are these indicators visible at night?
Direkt auf der Triebwerksgondel sind bei der Ju 52/3m die Treibstoffanzeigen montiert
The fuel guage is mounted directly on the engine pylons on the Ju 52/3m.
It is suggested that the indicator gives a reading in hundreds of liters (Liter x 100 written underneath).
Visible on the illustration below as Kraftstoffuhr ('Fuel Clock').
My Google foo fails me, but I did find this picture with a caption stating that it's a fuel gauge.
Zooming in on the image suggests that this is plausible
I also found this diagram of the wing fuel storage which shows the gauge.
To read it at night, pilots would use a flashlight.
I can't find any authoratative source. If I do, I'll come back and edit.