rb2013's post above about his experience with the 1959 Amperex pinched waist (I am assuming US-made? Probably 7L4 *9?) reminded me that I hadn't listened to my US-made 6922's in over 6 months. I have two versions, one 1961 (7L6 *16), and the other, 1964 (7L9 *4J).
Similar to rb2013, I am a "tone guy", so Siemens, Valvo and the like just don't do anything for me. Of these two US-made Amperex 6922's, I remembered that I much preferred the 1961. The 1961 had just a bit more warmth and body in the mid-range compared to the 1964, which is a bit leaner and drier. Vocals, especially female vocals were much more enjoyable and engaging.
So I rolled my 1961 this evening, Diana Krall, Live in Paris, and realized that I had forgotten just how good this tube is. While I am virtually certain that my 1961 tube can't compare to rb's 1959 tube, in my opinion it is significantly superior to my 1964 tube. At some point between 1961 and 1964, somewhere between revision 7L6 and 7L9, IMHO, the US-made Amperex 6922 changed for the worse. (I also have 1964 US-made Amperex 7308's, and these sound virtually indistinguishable from the 6922's manufactured at the same time.)
So for those of you like myself, who likely will never be able to afford the pinched-waist versions of the US-made Amperex 6922, I suggest to you that straight-sided (non-pinched waist) tubes manufactured no later than 1961 or 1962, revision 7L6, or earlier, are probably as close as we can get.