We can find it in several catalogs. Here, respectively, in the Gebrüder Schuster (ca. 1924-29), Matthias Hohner A. G. (ca. 1923-35), Meinel und Herold 1930, Meinel und Herold 1932, Vogtlandsperle (ca. 1923-35) and Max Adler (ca. 1929-32).
Here is the Gebrüder Schuster page :
If you look closely at the Max Adler picture (the last of the 6-series), it will appear to you that something seems to be written on the base of the mouth cap.
The flat part of the air duct cap seems shorter than on the other drawings.
So, are there two different German nose flutes ?
Well, I have two old flutes which correspond exactly to the drawings. The first one, with no stamp on the mouth cap was sold to me in Danemark as "a german flute". The second one, shows a stamp "Made in Germany" and comes from USA.
I don't know if one is a later model, or if the stamped one was destined to abroad market...