The knife is no bayonet, because it's missing the socket (the German bayonets had a ring on one side of the guard), is too short and has the wrong blade. This is a Grabendolch, the typical knife of the Sturmtruppen. It was a simplified Nicker (a hunting knife) bought at first privately by the soldiers, because the 10 inch long bayonet was of little use in the close combat of trench warfare. Later the Grabendolch was issued by the army.
I love the details in Erica's pictures: The belthook on the grenade only present on the WW1 model, the ripcord dangling from the unscrewed end of the grip ...
@Garrus: They show only grim determination, not Tanya's demented joy IMO!