Where lies the problem in thinking that NX-01 was exactly analogous to NX-2000, in a consistent and logical scheme of registry prefices?
Sure, our heroes may name-drop "NX class" and "the NX program" every now and then, but today's sailors speak of "688 class" or "DD(X) program" in quite analogous a manner, in a navy that firmly believes in naming the classes after the first ship christened.
In practical terms, NX-01 and NX-02 would be experimental vessels in the Enterprise class, quite possibly followed by two or three more prototypes before the groundbreaking class would be accepted as "no longer experimental". And the UESF of the 2150s does use class names deriving from the lead ship of the class, at least in the case of Neptune class and possibly Triton class as well, whereas there is zero evidence that they would have a custom of "lettercode classes" extending beyond the references to NX class. (Civilian ship classes from the era are a completely different matter, of course.)
Timo Saloniemi