NOS, or "New Old Stock", is a term used to describe something that is old, but never used.
This term is used in many many things, but especially watches.
Everyone wants to find a NOS Patek, or NOS Vacheron, etc.
The idea that you could time travel back in time, before a particular watch was considered "legendary", buy it, store it, and then transport back to the present. That is the allure of NOS.
And while to some degree, occasionally one does come across a watch that really is a NOS, most of the time is just BS.
And the reason why you have to be careful is because with NOS comes a much higher price tag.
As an example, I was browsing the web and saw what appeared to be a very nice Omega Pie-Pan Constellation. And as always, I wanted to verify if anything had be done to the watch (refinished dials are common).
But, the response I got was that if I wanted something untouched, this person had a NOS Pie-Pan constellation. But price wise, it started at $8000, more than 4-times the correct market value for this particular watch.
This was BS.
First, I would need more than just this person's "word" that it was truly NOS, and second, who cares?
If one buys a NOS watch from any era, and decides to wear it, guess what? It's no longer NOS, it's used!
Which brings us back to why vintage watches are so attractive: they're fantastic products, but the history that comes with a 40 year old Speedmaster, or a 70 year old Mark VIII, or whatever, is a huge part of the attraction.
So, unless you're buying a NOS watch just to store it (which is a silly thing to do), it makes no sense to pay too high a premium for NOS.
Instead, be patient and look for a watch that is in excellent condition despite its age, and buy that one.