I found one of these for sale recently, and I ordered it. My mom had one when she was flying in the late-'60s/early-'70s. It was always plugged into the socket where she had her morning coffee and book. I thought it was the neatest flashlight ever. Switch it one way, and the bulb in the clear lens lights up. Switch it the other way, and you have red light.
I’m not current, as I can’t afford to fly until I get rid of some debt. When I was flying before, I only flew at night while on training flights. When I eventually do get back into the air, and when I fly at night, I have a Mini Maglite with the red filter in. I also have three or four pilot’s penlights, one of which was dad’s and is in its original box.
So I have no need for a Sanyo NL-421. Still, it will be nice to have for the collection. If you happen to come across any more, let me know. I do like to have my duplicates! 
Never saw a need for red lights in the cockpit. Can’t see a damn thing with them. I use a headlamp with a soft white LED. It illuminates whatever I’m looking at without destroying my night vision.
I remember your flashlight. Use to have one. I feel your aviation pain. In the same boat. maybe we should switch to boats.
I always liked that flashlight.
It’s pretty archaic these days, and if I were to find one at a yard sale, I would most likely upgrade it with an LED light source and make it 5x as bright and much longer running.
Does the one you bought still have decent runtime? The batteries must be pretty much cooked by now…
It charges and retains a charge. I’ll probably try to figure out how to open it to re-cell it, if needed.
Negative. I’ve had a boat.
I have a super-bright red LED flashlight that I used to use for night flying and astronomy. It was a drop-in module similar to this. Very efficient since it produced red directly (not white with a filter). I could swap it for a white and UV module as well. Very handy!
A boat is a hole in the water into which you toss money. I’m not sure what an airplane is like…