shmuelyosef said:
I agree that the SN range from ~250xxx to ~300xxx are the best all around horns
I agree that there would be many, many fine horns in this range, but your chosen range is kind of perplexing to me, considering that around 272,xxx the Zephyr became almost a clone of the Super 20 with the same bell and all (except for the necks; the Zephyr's was overslung); whereas from 250,xxx until that number it had the slimmer pre-war Zephyr bell. In some ways I'm splitting hairs here, as any Zephyr made between around 1937 and 1957 can be a fabulous horn (and even those made after that aren't bad, at least up until the move to Eastlake).
I am speaking from a fair amount of experience, as I have owned no less than six Zephyr tenors from that 20-year period (1937, 1945, 1952, 1954, 1956 (two examples from that year that Elvis took the world by storm

) and have played about half a dozen others in some fine vintage sax shops. My conclusion: My current 1956 tenor outplays them all (though the 1937 example had a beautiful haunting tone). The '56 is fatter and louder than the '37, but still warm, full and smooth across the registers like all great King's are. Plus the intonation is nearly spot-on, something that is harder to find on many American horns made prior to the mid 50's. I don't know what happened as far as technology around that time, but folks (e.g. Bootman) talk about the intonation on Martin's dramatically improving around 1955 or '56 as well. In short, this horn is the best King I have ever owned and plays better than a Super 20 I used to own.
There are some that say there is a clear demarcation in the "mid 50's" in which the Zephyr took a nose-dive in quality. This is yet another example of how one statement made by a guy who made a website with pictures of saxes on it can be repeated over and over again and eventually be accepted as fact. In reality, if you check the official H.N. White website, you will see that the Zephyr didn't really become a "student" horn (if in fact that's what it would be called when compared to the cheap stuff coming out of the Far East today) until 1958, when the double-socket neck was eliminated and some other aspects may have been cheapened as well. My '56 still has the double-sleeve neck and is a heavy, durable horn with relatively smooth, quiet keywork.
Good luck in your search! If you are big guy like me (6'3" with 37" sleeves and big hands, you will find the King's are more comfortable ergonomically than any other vintage horn). They just feel good in the hands.