rgp4544,
I use an F04 on my early-generation (1?) L1-RD with great success. It thins out the center hotspot and diffuses the beam a little broader for night navigation work. I'm very pleased with the result. BUT!!! I do NOT know how effectively it will work with the new generation pencilbeam lights. I understand that the new L1s, E1Ls, E2Ls, and KL1s all have the new Luxeon III pencilbeam/distance LEDs. When I tried an F04 on a borrowed "new-generation" KL1, it seemed to be ok but less diffuse than the older version. For that reason, I have yet to get one of these new lights since I prefer floodlight for all close-in work. Hope this helps.