The short tube on the front of the standard Luftwaffe 300 litre drop tank was a channel into which what was essentially a 'stick' could be inserted -- as a visual indicator to the pilot that the drop tank was still in position. [The stick was long enough to be visible forward of the wing leading edge, from the cockpit.]
Some period photos show just the tube (before the indicator was inserted), some that look like a longer tube are in fact with the stick in place...and some clearly show the short tube broken off, either through accident or handling. Occasionally photos of jettisoned drop tanks can be seen with the stick (or part of it) still in place.
As a rule, photos of a/c with a drop tank mounted under the fuselage may show the tube, but no stick...since it couldn't be seen by the pilot in that position. [And as some photos clearly show no tube at all, it's reasonable to assume that tanks manufactured exclusively for under-fuselage mounting would have dispensed with that un-needed manufacturing step altogether.]
Here's a discussion on another forum, if you're interested:
https://ww2aircraft.net/forum/threads/luftwaffe-drop-tanks.45449/
Cheers