My wife just got me my retirement gift - Postal Meter serial number 41245XX and the barrel is marked underwood 1-43. Seems like a good gun, but the gas piston nut was loose. Gonna clean it up and put some blue Loctite on the gas piston nut to secure it before I shoot…unless someone is going to tell me I am off base.
If the nut is staked in place but turning a little it's OK. But if the nut can come off then it needs to be tightened down snug but not too hard as you can crack the gas housing and if that happens then you replace the barrel. If you have the ability to stake it in place then do that. Loctite is not a good idea on the castle nut of the gas system. Your carbine would date to about January 1944. If your barrel date is 1-43 then it's a replacement and I would guess during a rebuild which is pretty much standard for all carbines.
Gas nut was able to be removed completely. Read on the CMP forum (I think) that early carbines were not staked, and that blue Loctite could be used in lieu of staking. Gotta get a gas nut wrench first.
If you get one of the castle nut wrench's be very careful with it. The nut is just turned till nice and snug and no more then that. It may look like it can go into the gas cylinder deeper but don't try it as it's most likely bottomed out. Too much pressure and the gas cylinder can crack and your SOL then. The blue Loctite is not needed and it's best to stake the nut in place once it's tight.