The reason for the leather inserts is the knife and fork would poke holes in the canvas pockets. Besides, it could get rather unsanitary as well and was better to have a removal insert. That is if you didn't keep your equipment clean like you were suppose to. That is also why you don't usually find a leather insert for the spoon. Spoons are great for digging escape tunnels but won't usually poke a hole in your haversack utensil pocket.
Over time the original leather inserts would shrink needing replacement. I've got some old ones that are really stiff and unusable now.
Oh, also the pockets are always in the meat can pouch, but in the WWII bags they are in different spots then they are in the WWI version. Off hand I forget which was which, but you can easily figure it out. The older bags (model 1910) have button fasteners the newer ones (model 1928) a strap and buckle like yours does.