After some further research:
The rifle originally belonged to the Telegraphen-Abteilung of the IX. Army Corps and had the unit stamp:
T.A.IX. 7
When the war broke out and mobilization was ordered, the IX. Army Corps was sent to the Western front, but had to provide some soldiers with equipment from their active formations to form the Reserve units. That is when the 2.
R.D. was added to the stamp.
In WWI and WWII it was common to form new units with some fully trained soldiers from active formations.
As mentioned earlier, the IX. Reserve Corps had more Corps units than normal Reserve Corps. With the long coast line and the islands they had to protect, they also needed more communication units.
But only for a short period. August 23rd 1914, the IX. Reserve Corps was sent to Belgium.
The 17. and 18. Reserve Division of the IX. Reserve Corps committed the war crimes in Leuven.
https://en.wikipedia.org/wiki/The_Rape_of_Belgium
The new rifles for the 2. T.A. got the unit stamp with the 2 between the IX. and the rifle#, which is according to regulations.
It can not be from the 2. Reserve Division, because the German Army did not have a 2. Reserve Division in 1914.
It can not be from the 2. Reserve Dragoons, because they were part of the III. Corps.
Thanks