Replying to Tyrone's question:
When you look at the way how Kawai uses the name "Millennium III action", then you see that this name does not include the key sticks or the hammers (and that usage of the term "action" is common in the acoustic world). While here in the DP world we usually call the whole assembly of moving parts, from keys to moving weights, the "action" (and I think that is where your confusion stems from). The name "Millennium III" refers to the mechanical bits between
the key sticks and the hammers (i.e. the wippen, jack, repetition lever and spring, etc.). At least that is what I gather from Kawai's websites. They always talk about the Millennium III action and the keys and the hammers separately.
Of course Kawai's smallest grands have shorter key sticks than their concert grands (and so has the NV10). I don't think Kawai is engaging in any deception there. The smaller/cheaper grands also have different hammers. But the bits in between, those bits that the name Millennium III refers to, seem to be the same throughout their whole line, from NV10 and GL-10 up to the SK-EX. (That is, if we are looking at the mechanism, and the parts involved, for a single key. The whole assembly, for all 88 keys, differs between different pianos - as Johnstaf pointed out - because they have different requirements for the overall layout to fit into the respective piano and to mesh with the other parts of the piano.)
So to attempt an answer for your question:
Kawai has this Millennium III action which, mated to different keysticks and hammers, and in various assembly configurations, they do indeed use in all their current acoustic grand models, from the lowly GL-10 and NV10 up to the SK-EX.
Yamaha does not seem to have such a "named" action that it uses in all of its models. Instead, in addition to the variations in keysticks (and hammers), there also seem to be variations in the core action between Yamaha acoustic grand models. And the AvantGrands may even have their own core action that isn't used in an actual acoustic grand model.
As said above, this claim does not apply to the key sticks (and pivot length), but only to the core action. So from this claim, you should not infer that the pivot length in the NV10 is the same as in an SK-EX concert grand (which it isn't).
But this statement:
Yet I am reading in this thread the claim that the action of the N1X is only in Yamaha baby grands and not in Yamaha's concert grands.
was made specifically with the keysticks (and the pivot length that results from the stick length) in mind. And that part is true. The N1X action has keysticks of a length comparable to a Yamaha baby grand, not a Yamaha concert grand. It also
seems to have a core action that is not the same as the core action in a Yamaha concert grand
Does this mean the N1X's action is inferior to the NV10's Millennium III action?
On paper, maybe (and arguably). Because, yes, the NV10's core action is the same as that of Kawai's concert grand, while that is not true for the N1X. And if we again expand the comparison from the core action to the keysticks, the NV10 has
a longer pivot point than the N1X.
But in real life, with regards to what you feel when playing it, I don't think so. It isn't "inferior", it is only "different". And it depends on your personal preference which one you like better.
(If one want's to be absolutely nit-picky, one could say that the Millennium III is arguably less susceptible to humidity fluctuations, as it uses less wooden parts - but again, I doubt that this would make a difference to most owners.)