Sounds like it was made for or by Gustav Genschow (=Geco) Berlin-Treptow. They were active from the late 1880s
I've got one of their .22 youth rifles made in the 1920s...
Owned by a Jewish family, it's assets were apparently taken over by the Nazi government in the late 1930s.
There is currently a company linked to the name. Details at:
http://www.genschow.com/engl/english.htm
Marc
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Igitur si vis pacem, para bellum -
- Therefore if you want peace, prepare for war.