Perhaps WWI, I had one when I was a kid. Bought it in a surplus store. They had a bin full of them. Mine was a double edged dagger, and I though it was American, but these style of knives were used by a number of different countries. If I remember correctly I think there was a model date on my knife, and I think it was 1917. Does your knife have any German writing on the blade? Or a model date?