Well, we do know that in the 24th century SF uses NX as a designation for experimental class of starships.
Not quite. The United Earth Starfleet uses it for an
operational class of starships. Which happens to be an
explorer class of starships, supposedly the first ever for the UESF. So there we have an alternate explanation for the letter X in combination with the number 01...
In contrast, one would assume that the UESF had by that point had plenty of experimental ships, some of them one-offs, some prototypes for the many ship classes we witnessed in UESF use. The Warp Five Explorer should have been something like NX-72, then, rather than NX-01!
Besides, the NX class from the mid 22nd century might have been retired pretty soon after the founding of the Federation with but a few being built overall
The NX-01 design might have proven to be a dud, really. If she were the first-ever operational incarnation of the Warp Five drive, the second incarnation would probably be worlds above her in every respect. It wouldn't be difficult to accept the idea that not only NX-01 but the entire class would be retired by the time of "TAtV", and that it would have lingered that long only because the Romulan War called for it to.
(Incidentally, if NX-01 were formally considered to be "
Enterprise class", as the systematic designation would go today and supposedly in the UFP Starfleet, and if it survived past 2161 and transferred from UE to UFP, then we'd have a nice explanation as to why none of the UFPSF
Enterprises have been the first ships of their respective classes: the class name has already been "used up"!)
Timo Saloniemi