Tube sellers

Often it means NOISY old tubes that nobody wants.

To me,NOS is just what it says NEW old stock. I agree with you that a lot of tubes being sold as NOS are probably used and test to new parameters but they are not NOS. You have to find a tubemeister you can trust. How do you do that? I have no idea,I guess faith will have a lot to do with it !!!! I have a lot of faith in mine.

NOS means just what it says: New Old Stock.
How much you believe the tube is actually NOS depends on the tube.
NOS Mullard EL34? Maybe not.
NOS CBS Hytron 807? Good chance it is. And many rectifiers.
Just my opinion of course

There are NOS tubes out there, but the supply is dwindling fast. It is important to buy from a reliable seller. I have had good service from Kevin Deal at Upscale Audio. NOS tubes come and go and when there are fewer tubes left the price goes up until they are gone. If you snooze you lose.

'NOS' tubes are one of the bigger problems we have with our preamps. It is very difficult to find NOS 12AT7s that are as quiet as actual new ones. As with any high-performance tube equipment, you can't just buy tubes off-the-shelf and have them work 100%. Usually you have to hand-pick them.

So in practice, when people have bought NOS 12AT7s for use in our preamps' phono sections, they are often buying tubes that are significantly noisier than what we regard as acceptable- then they assume that the preamp is noisy! Before we even audition a tube, we start by grading it in a tester. It must read about 80% higher than the 'passing' value to even be considered for use. Incidentally, that number is typical of 'testing like new' for 12AT7s, anyway...