German WW1 Field artillery dog tag
Issued to a soldier named "Hugo Reising".
He served in Ers. Abt Fd. Artl R11 - Res. Feld Art 13 (Reserve-Feldartillerie-Regiment Nr. 13), which was a field artillery unit.
There is a lot of information available so it would be rather easy to do further research.
Dog tag is from around 1914 and it seems that the soldier was only 16 at this time.
This is the earliest version, later version from 1917 and after had the slots in the middle so you could break it in two if the soldier died.