Any idea what this tool was used for?

[USGI]

Dunno for sure without dimensions, but the business end looks to be the correct shape to remove/ replace the gas cylinder. Can’t figure out what the spring is for though, so probably wrong

My thoughts exactly, and I think the "gas system" was mentioned in the description. Maybe the spring would sit up against the receiver and maintain pressure
(and alignment) while turning the nut? It's Knurled, so you could turn it with your fingers, or use a bar in the hole if necessary. There could be another piece for "chasing" threads that fits into the end of the tool for alignment, and having lugs so it could be turned in the same manner as the nut. I'll see if I can find and post a link to the auction. Need to check the rules to see if that's OK first. - Bob

https://www.ebay.com/itm/WWII-M1-Car...p/333499034189