The acronym "NOS" (New Old Stock) is overly used by many, especially on eBay. This means the tube has never been used and tests at or above new values with no issues, shorts or gas, etc. Also, test results should be on a recognized tester that is calibrated. The tube should come in it’s original box, however sometimes that isn’t possible. Bulk pack tubes were packaged 100 to a single box. Many years ago I acquired some NOS Brimar tubes that were the remainder of a bulk pack and I did get that original large box. Many sellers label used tubes "NOS" if they test at or above new values, and most of the time what tester was used is never cited. This is very misleading, since it’s actually a used tube that has good test results if done on a recognized calibrated tester.
The term "matched" gets used loosely as well. Is the internal structure of each tube the same? Do the date codes match? How about test results, within 10%, 5%, etc and how does that relate to the sections of each tube, then to the pair? What type of tester was used and was it calibrated?
As others have said, it’s best to buy from a trusted seller. Otherwise before diving in, you need to know the water is clear or be able to detect just how cloudy it is. Sometimes on an extremely good deal that was just ever so slightly cloudy, I have taken a chance and everything was OK.
Now look back at those 2 pair of tubes and see if you can tell how different they really are.
Another reputable seller for new and NOS is TC Tubes. He also offers a testing service for your tubes with what is considered a reference tester, the Amplitrex.
