Ers.Kp.Gren.Ers. Batl.78 EKM (Aachen)

The dog tag or 'Erkennungsmarke' shows signs of wear. The bataillon was formed in the summer of 1939 as Infanterie Ersatz Bataillon 78 and it trained and supplied soldiers for Infanterie Regiment 78. In July 1941 the ersatz bataillon moved to, Venlo (the Netherlands) and in November 1942 it was renamed into Grenadier Ersatz Bataillon 78. 
In August 1943 the bataillon moved back to Germany, Dusseldorf. In July 1944 it was renamed in Grenadier Ersatz und Ausbildungs Bataillon 78. In October 1944 the bataillon was mobilized and became part of the 49.Infanterie-Division and together with other units it was used to defend Aachen against the Amercians during the Battle of Aachen. After that battle it fought again in Würselen. The units that survived were used to reinforce the 246.Volks-Grenadier-Division.
The 'O' on both parts of the tag stands for the soldier's bloodtype. 
Interesting EKM.