Natural materials equals non-metallic natural substances. Except arrow points and non-mechanical release components may be metallic. I think this was put into place to avoid the use of aluminum arrow shafts with the claim of aluminum being a natural material. So, the bows and arrows don’t necessarily have to be made of wood. The fletching on the arrows doesn’t necessarily have to be from feathers. It just has to be a natural material. Examples: Bone, wood, horn, bamboo, sinew, vegetable fiber, leather, rawhide, gut, etc. I’ve seen velum (fine rawhide), Horn, and hand made paper used for arrow vanes.
Nocks have been made from all sorts of things: horn, bone, self nocks reinforced with sinew or silk or dense hardwood, etc.
Alan