I got such filter for a common GNm 40 flashlight. To my surprise... The filter is obviously from the same material, hue and thickness as the cover of the reflector: slightly yellow, good visible when looking through the edges. In general reflector covers vary in colour. Sometime, as with Metz, even at the same model over time. At some models the tube has a slight yellow hue. From all this I got the impression that manufacturer of flashes coped with UV-radiation, either at the stage of the tube or the cover, at least from the 70s on. When would a doubling of UV-filter density be necessary? I can't do UV-meterings, thus I would like to hear of your experiences.