Eh, but NCC-1700 was shown on the status chart on screen. Unless it's the prototype, and unless Mr. Jefferies's comments allowed this out for the prototype, it's hard to reconcile. Better than the rationalization I've sometimes seen - such as it being the cadet class from the Enterprise. Forgive me - do you mean to assert then that the TOS version of the ship is the 16th design, and the 17th design is how she appears in the movies? I must be missing something because that doesn't quite work. Agreed. I once presupposed that Class 16 was a design that looked almost like the Constitution class - so much so that you almost couldn't tell the difference with a simple swap of the nacelles and a couple other details. All supposition, of course. But ignoring the 16xx registries as Constitutions makes more sense IMO. You know, the extensive refit of the Enterprise as seen in TMP is kind of an oddball overall. Jefferies presupposed that the engines at least were modular and easily swapped. We know, of course, the real-world reason for the extensive external refit of the Enterprise... but it's really the only such refit we've seen; one where the ship's exterior changed dramatically. The only remotely similar cases I can think of are the three-nacelled D from "All Good Things" and the Enterprise-E from "Nemesis"... the former being in an anti-time future, the latter being so minute we might just ignore it and assume the ship looked one way or the other all along. We can presuppose that older designs seen in TNG onward utilized newer tech in a way that is not visually discernible other than the odd random nacelle glow here or there... or maybe they're really not that different internally than the were in the movies era. In any case, the TMP refit seems to be the only case we've seen where, again the design changed externally in a very dramatic way... which makes me lean towards thinking that such refits are uncommon, and the reasons for the Enterprise undergoing one are somewhat obscure.