St. Hubertus is the Patron Saint of the Hunter. The jist of the story is he was hunting instead of honoring Good Friday as his devout wife asked him to. Stories have him with a bow or a crossbow and when he was about to let loose the arrow, a cross appeared between the deer's antlers. And then God spoke to Hubert.
Hubertus was a trade name/trademark of the Immann Meffert. I can't say that it was filed for like Sauer's "caveman". Immanuel Meffert began in 1839 in Suhl and his sons Justin & Richard followed by Justin's sons Bruno & Gerhard in 1934. Meffert made drillings, and dural drillings, side by sides, O/Us, Vierlings, air rifles and many more as between the Wars production topped 1000 hunting guns per year. They focused on practical quality and not on embellishment so an engraved one won't be found all the time. GGCA(
http://www.germanguns.com ) may have a reprint Meffert catalgoue. There's a couple Meffert articles in the GGCA Winter 2002 Journal, I believe.
Where are the cocking indicators? Meffert had some unique actions like the Treff and unique safety devices. Meffert was noted for having lightweight longarms. From the marks you describe such as the "Nitro", vulgar fractions and being void of a date stamp, yours was made during the 1912-WWI period.
Kind Regards,
Raimey
rse