But if all that is untrue, NX did NOT indicate anything about an experimental vessel, as you suggest. Well why wouldn't it?...And if so, what's the meaning of NX as used on Enterprise? NX class? Random letters-class?
Personally, I think that "NX" did indeed initially stand for an experimental vessel just like it does in the Federation Starfleet, but that somebody decided to change their mind about that later on.
And why "NX-01?" Simple. Because the Dauntless
, which was supposed to be an experimental vessel (even though it turned out to be fake), had the registry NX-01-A. To get away from the whole "TOS Enterprise was the first Enterprise" thing, they probably were going to rename the NX-01 the Dauntless
after the formation of the Federation. That's just my opinion, though.