The problem is that the registry numbers mix metaphors. The US Air Force puts X and Y into its aircraft designation system to show planes that exist to test concepts (X) or planes that are early versions (Y) that are intended to be improved upon in production. Unless the Defiant were a one and done deal, like a plane, it would be NY, not NX. On the other hand, the US Navy doesn't use hull classification to denote experimental sea craft. There is no DEX or CCX. It's simply too expensive to build a full ship solely for the purpose of experimentation. It would simply go into service.
Quite so.
It would be easy to argue that both NX-2000 and NX-74205 were intended by Starfleet to be "X-planes" rather than "prototypes": the former was a modification of an existing capital ship for the sole purpose of testing the transwarp drive, the latter was an experiment on how to produce an anti-Borg platform. That the latter then actually spawned a class would be wartime convenience, and peacetime ship construction would instead involve actual prototypes (possibly with NY numbers) leading into series production, full aircraft style.
We basically never see other NX ships on screen, and the idea that they would be prototypes for ship models that later see series production is more a fan convention than something inherent in the material. Sure, Okudagrams and the like list an NX-registered
Galaxy or
Constellation, but those are obscure things, ambiguous and comparable to the Porsches and ducks on the MSDs.
(The United Earth Starfleet use of the letter X is likely to be more logical and more in the naval convention: Archer's ship is introduced as the very first UESF deep space eXplorer, so NX-01 makes great sense there. And since we know that UESF gives its ship classes perfectly ordinary class names (we hear
Neptune and arguably
Triton), we might also assume that Archer's ship is of the
Enterprise class, in which case her also being "NX class" establishes NX as a mission-descriptive thing, comparable to a ship today being CVN class or DDG class...)
As regards the letter-suffix ships of DSC, statistically it would appear that
all the TOS through VOY era ships we've heard of will get the letter! That is, it really doesn't take much...
Timo Saloniemi