Afaik at the time the rifle was made, the only .22 cartridges known in Germany were the 6mm Flobert and the .22 short. But don't think too much about an "apparently .22lr chamber". Most of the pre-war licence free, nominally "6mm Flobert" single shot rifles are chambered very generously, maybe for the longer 6mm Flobert shot cartridges. They readily accept and shoot the .22lr. And don't forget the possibility of a later rechambering job, done with some household tools and bypassing the proofhouse. BTW, 6mm Flobert rifles were apparently not rated as "firearms". I have seen several Zella_Mehlis ones, made during the interwar years, without any proofmarks.