17.Kp./Grz.Inf.Rgt.121 Erkennungsmarke

The tag or 'Erkennungsmarke' shows overall normal signs of wear. It belonged to a soldier who served within the 17.Kompanie Grenz Infanterie Regiment 121. This regiment was formed in November 1938 and was part of the 50.Infanterie-Division. It saw it's first combat during the invasion of Poland that took place in September 1939. After Poland it moved to the West and was renamed Infanterie Regiment 121. In May 1940 it participated in the invasion of France. After that the division and her units moved to the Balkans and operated in Rumania and Greece in early 1941. In June of that year it participated in the invasion of Russia and operated in areas like the Krim, Kuban and Sewastopol. In October 1942 the regiment was renamed into Grenadier Regiment 121. It was destroyed in 1944. 
The 'A' on the back of the tag stands for the soldier's bloodtype. 

Some very detailed information on this unit can be found online.