I have no idea about this one.
The cartridge is a paper case with a glued seam. There is an iron washer at the bottom to close it all together. The iron base is riveted to a metal bar (probably iron) which is magnetic and appears to go into a 4mm percussion cap resting against the base of the bullet.
Bullet + paper case diameter: 8.3mm
weight: 8 grams
If I find another I’ll be sure to section it!