I think the main point here is that no-one is intending foul play. To me, the most plausible answer is that it is some kind of Omega prototype rather than a put-together.
The case looks like the ones used on the
ultra-thin Constellations of the seventies, but apparently lacking any markings.
Had it been a Franken, those markings would have been present. Had it been a fake, I
seriously doubt that
a) a rather high-end, unknown movement would have been chosen.
b) the dial would have been this good.
c) it would have been this particular case shape. It has to one of the least collectible Constellations out there.
Let us not forget that this was a time where Omega tried out a lot of new stuff with varying success. IMO, the existense of another one with the same dial type speaks volumes.