Steel stamp (marker) of Waffenamt (or WaA) of the Third Reich. Letters WaA and marker's number 518, wingspan of the eagle 5 mm. The most often this design can be found on German weapon parts, i. e. Mauser bayonet.
Handle height is about 10 cm. Hardness of the stamp is GOST 25727-83 from 55 to 62HRC. You may put it on steel under 43HRC. You shouldn't use the stamp on harder steel or on hot steel. Check hardness of steel you work with before use the stamp.
How to use the stamp? Put your metal detail on a durable base (like a massive table, forge or rail). Lean the stamp against the detail and make a strong springy hammer. Never hit twice!