A family friend recently gave me this large trench knife, which was passed down to them by their father. Their father was a WWII vet who served as a ball turret gunner in Europe. She claimed that this knife was something he brought back as a souvenir from the war. However, it has no stamp indicating its manufacturer, so I'm skeptical that it's from the war and not just something he picked up as a novelty years later. Take a look and let me know what you guys think. http://imgur.com/a/KE8Db
You might have better luck at r/knives or r/swords. WWI saw several "knuckle knives" which adapted a knucklebow guard with spikes and/or finger-holes for punching, most famously the US 1917 trench knife, but what you have here is an unusual variation I've never seen before.
It's pretty definitely not US military issue. However, that does not necessarily imply that it was not actually used/carried by the gunner. He could have picked it up at a local market nearly anywhere he was stationed. As noted below, it is vaguely reminiscent of one of the US M-1917 trench knives, but is certainly not one of those.
You're not likely to find answers on this sub as it seems to be a one-off made from parts, and quite a rough one to be honest.
Note the unevenly sized, poorly finished welds holding the knuckle duster to the handle, the seemingly hand-made checkering on the dagger's handle and the tacked on pommel spike, which is not covered by the scabbard, meaning it's more likely to hurt the wearer than the enemy.