The knife looks like a bayonet but there does lot look to be any mounting mechanism or spring loaded catch.
Is there a slit in the back of the grip to engage in a rifle mount?
If i was to guess the blade looks to be a German Faschinenmesser but the hole for the barrel in the cross guard is putting me off this idea
http://www.pickelhaubes.com/bb/viewt...hp?f=36&t=6523
gives you an idea of this knife type, used by troops for clearing brush and cutting small branches in 18oo's, if you scroll down in the above link you will see the amount of variations in this knife type there were
regards
Ken