I paid well for this at a bit under 300, but it's finally in my hands. This is an M1 Carbine Armorer's Tool. It's specific use was to remove the gas nut and rethread the screw threads in the gas port (could also clean them out quite well). I got it because it'll make a nice addition to the collection... and because I frankly needed it. My latest M1 pickup had slightly messed-up threads, a loose gas nut, and a sticky piston.
So, now with the correct tool in hand, I should finally be able to get her range-ready. If anyone needs to borrow it, let me know. I'll lend it out for a reasonable fee.