"Found in a river in Manchester UK (white number painted on) – seems to have a light bulb but no electric mechanism inside, knob on top pushes down and twists, then releases with a loud click. Engraved “Patent No. 217135” with no results."
A: "Looks to be an Eveready bike light of some kind. I recall something about bikes and cars in ww2 having that kind of deflector to reduce lights in blackouts (so it only shines directly below the vehicle, not out into the road so much."