The two scouts and the dreadnought mentioned over the Epsilon 9 radio chatter in TMP had registries of 6XX and 2XXX respectively. This info was taken directly from Franz Joseph's Tech Manual, which inferred that at the time, registry numbers were based on the type of ship, not when it was built chonologically (all scouts had 6XX numbers, all dreadnoughts had 2XXX numbers, all heavy cruisers had 1XXX numbers, and all tugs had 3XXX numbers).
In TWOK, the Reliant had a registry of 1864. I believe this was done to show that she was a bit newer than the Enterprise, so one could assume that this was the start of registries being chronological to production date.
However, in TSFS, the brand-new Excelsior was NX-2000, clearly lower than the Entente from TMP. I rather doubt that whoever gave the Excelsior that number was even vaguely aware of FJ's work, and that the "2000" was simply used because it was a "big" number for a "big" ship (this was only 1984 after all). Conversely, the Grissom's NCC-638 was a small number given to a small ship. I truly believe that the size of the ship was the only rationale for this, so one shouldn't really try to over-analyze the numbers. By the time of TNG and DS9, there was far more of a cohesive, chronological-based registry system (3XXXX for the Mirandas, 4XXXX for the Excelsiors, 5XXXX and 6XXXX for the pre-Galaxy class-style ships, and 7XXXX for the Galaxy class and newer classes...although how the TNG Oberths with insanely high 5XXXX registries got in there I'll never know...I even wrote a whole essay about that

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