It's Nr. 20... from 20th Division. I've to admit that this also troubled my mind at first, but I looked it up at Wikipedia and the following is the result:
On mobilization in August 1914, most divisional cavalry, including brigade headquarters, was withdrawn to form cavalry divisions or split up among divisions as reconnaissance units. Divisions received engineer companies and other support units from their higher headquarters. The 20th Division was again renamed the 20th Infantry Division and its initial wartime organization was as follows:[9]
39. Infanterie-Brigade
Infanterie-Regiment von Voigts-Rhetz (3. Hannoversches) Nr. 79
4. Hannoversches Infanterie-Regiment Nr. 164
Hannoversches Jäger-Bataillon Nr. 10
40.Infanterie-Brigade
2. Hannoversches Infanterie-Regiment Nr. 77
Braunschweigisches Infanterie-Regiment Nr. 92
Stab und "1/2"-Regiment/Braunschweigisches Husaren-Regiment Nr. 17
20. Feldartillerie-Brigade
Feldartillerie-Regiment von Scharnhorst (1. Hannoversches) Nr. 10
Niedersächsisches Feldartillerie-Regiment Nr. 46
2.Kompanie/Hannoversches Pionier-Bataillon Nr. 10
3.Kompanie/Hannoversches Pionier-Bataillon Nr. 10
On another forum this explanation was questioned, but I can see this as the only logical possibility... Maybe our experts can figure this out?