4.Kp./Inf.Rgt.21 EKM (17.Inf.-Div.)

The dog tag shows normal signs of wear. The regiment was formed in January 1921 as 21 (Bayer).Infanterie-Regiment and was part of the 100.000 Reichswehr force.
In October 1935 the regiment was renamed into Infanterie Regiment 21 and became part of the 17.Infanterie-Division. 

This division was put into action during the invasion of Poland in September 1939. After Poland it participated in the invasion of France where it stayed until Mai 1941 before it was transported to Poland to prepare for the invasion of Russia. 
It fought in areas like Brest-Litowsk, Bialystok, Gomel, Juchnow and Mius. In October 1942 the regiment was renamed into Grenadier-Regiment 21. 

In February 1944 the regiment fought together with units of the 3.Gebirgs-Division in the area of Apostelowo, Russia. In the last stage of the War (March 1945) the remnants of the regiment where used to form Grenadier Regiment 95. 

The 'O' on both parts of the tag stands for the soldier's bloodtype. 
Nice tag of an unit with a rich history.