Bad news friends. There is something wrong with the light. I swapped parts with my known good A2 and something is not working in the 4-flat parts list. I thought I had narrowed it down to the tail cap, but it seems that the regulation in the 4 flat is now allowing the bulb to turn on. A known good bulb, known good (fully charged) batteries, and a known good tail cap in the 4-flat body is not turning on the bulb at all. The 4 flat tail cap in the round body with a known good body and same batteries is not turning on the bulb in the round body either. So, I am thinking something happened to this light, maybe over voltage?
Is there anything I can do to make the 4 flat work again? Can I scavenge parts from the round body and put them in the 4 flat, and then send the round body to SF and have them fix it? If I do send it to SF, they are going to give me a new Aviator in replacement aren't they?
ETA: I finally narrowed it down to the tail cap on the 4 flat. I cannot get it to work with either bulb in either light, but swapping heads and bulbs does work in either light. Whew. Hopefully SF has a spare A2 tail cap laying around, or I can get someone here to show me how to dismantle and trouble shoot the tail cap on the 4-flat tail cap.