I have found these relics myself, and wondered if anyone ever saw these charges before.
They are the same in size and material as the average 1kg sprengbüchse.
Big difference is that this charge appeared to have only two fuze wells, soldered in a different way then a
"normal" charge.
On both the side and the base, the container is marked "FZ1". I have one base which is marked 44, see on the right.

The three 3kg covers in the photo below were found on the same location.


The photos below show the same sort of charges.
These might have been signalling charges, FZ might mean Farb-Zeichen.
I hope someone gets me more information about them.
Let me know.

Some more information came from Jim.
Apparently these charges were meant to destruct aircrafts and were Luftwaffe issue. I can imagine that "F" stands for Flugzeug (aircraft). The "Z" is sure to mean "Zerlege", which stands for destruction.
Thanks Jim.

Photos below © Joachim.