The screw mount Lens Hood from Leica is compatible with the Lens Hood for M 28mm f/5.6 Brass Ano. The hood is designed to block stray light from the sun or other light sources, and to prevent glare or lens flare.
The black anodized lens hood can also be used to protect the lens from accidental bumps or scratches. In the top left corner there is a side window for filter adjustment when using a circular polarizer or variable neutral density filter. The hood can be a spare, or a replacement item for what is originally included with the lens.