RoTuN,
Your fine old Meffert drilling was made before 1912, since the bore(not groove or bullet) diameter was expressed in "gauge"( 69,49 ga.). After this time, it would have been marked 10.41mm and whatever the case length is, would have also been shown. The fact that it carried any proof mark, means it was made after early 1893, when the proof law became effective. To determine the nominal caliber of the rifle, it will be necessary that you either make, or have someone else make a chamber cast, and "slug" the barrel. My "gut feeling" is you will find that it is chambered for one of the 10.75 LK express or Drilling cartridges. I do not believe it is chambered for one of the 43 Mauser variants, as the chamber doesn't show a significant shoulder. Once we determine the nominal caliber, if you want to load ammo for it; some of us here will be happy to help you.
Mike