STAMBTTR.H.FL.E.U.A.ABT.280 Erkennungsmarke

The dog tag or 'Erkennungsmarke' shows signs of wear. The bataillon was formed in May 1941 in Itzehoe as Heeres Flak Artillerie Ersatz Abteilung 280. It trained and supplied soldiers for different Heeres Falk Abteilungen. In March 1942 the unit was moved to Denmark. In April 1943 the ersatz and ausbildung unit became one unit. In November 1943 the abteilung was part of the 160.Reserve-Division, stationed in Denmark till the end of the war.
A Stammkompanie was a kompanie that trained the soldiers. Often the soldier of this kompanie were soldiers who had frontline experience and/or were no longer fully usable at the front due to injuries. Of course this changed later in the war as there was a shortage of soldiers. 
The 'O' on both parts of the tag stands for the bloodtype of the soldier. Still a small piece of chain is attached. Probably for a talisman/lucky charm.

Heeres Flak dog tags are not easy to come by.