Mike, I'd still guess that the 9mm tube was designed for some Flobert cartridge. I know that in Austria that Flobert tubes could forgo the preliminary test and be subject to the final proof, and I think the same was allowed in Germany. I shoot a 9mm Flobert a great deal. Where is the striker hitting the cartridge? Most of the early Floberts were paper w/ a metal base.

The diameter was 8.8mm so measure the diameter, directly or indirectly, and compare.
Kind Regards,

Raimey
rse