Dear all,
Thanks for your interesting comments! I have to admit that I don't know the Fi 156 that well. With "aerial" I meant what turns out to be the graduated tube. I haven't noticed this on other Fi 156 before but probably just because I haven't studied this aircraft in such detail.
I knew that the rectangular tubes are quite unusual and I think they are indeed related to the container which I still think is a fuel tank. I went through my old lwcollectibles site and found 3 more photos of Fi 156 with fuel tanks:
1st one
2nd one
3rd one
The one in the 1st photo looks very much like the one on the photo at
www.battlegroundrelics.com
Cheers,
Roger