This trenchknife / bayonet was produced by Robert Klaas & Cie (see the manufacturers stamp of the 2 birds)... Most of these were produced by Franz Koeller & Cie, Solingen-Ohlings and Gottlieb Hammersfahr & Cie, Solingen.
I think it's a private purchase for a few reasons:
1) there is no inspection stamp present.
2) the leather of the strap is of finer quality than normal (mayby that's why it has a period repair...)
3) there's a button hole at the back of the strap, which I think is not standard
Nevertheless it's a very rare item, certainly in the original color...