ConElPueblo
·It doesn't matter what piepan dials from other brands look like, for Omegas, it does contain a distinct 12 straight edges for the minute ring. These edges are also the edges of 12 flat sides beyond the minute rings.
The other Omega examples you posted all don't have the 12 straight edges or 12 flat sides. All have a pretty much continuous convex shape, hence they're called dome dials. They don't have the same look as piepan dials from other brands either.
That's my definition as well (bold part). That said, I don't think Omega themselves ever made any official wording on this subject, as far as I know it's all just a matter of what the community as a whole has decided on.

