Here is a photo of the handle of an M1 Rifle Bayonet - the same type found in the lake behind the Percy home.
Note the Pommel - that portion of the handle to the left in the photo. It has a roughly triangular shape when viewed from the rear. Normally this part attaches to the bayonet lug on the rifle, while the hole in the hasp on the forward part of the handle (right in photo) goes around the rifle barrel.
The bayonet could be used to stab, slash (if sharpened) and bludgeon. Usually, only the point would be sharpened, and the long blade sides left dull.
Although meant to be used with the rifle, the bayonet could also be used as a combat knife. The blade on this model was 10 inches long.