For some reason, many watches imported to the USA in the mid to late 1950's tended to have only "Swiss" on the dials. I have several LeCoultres and a Rolex marked this way, as well as a no-name Globemaster. Kyle's Globemaster that he recently sold was marked like that as well.
Many times this indicated that while the movement and dial were made in Switzerland, important work such as adjustments were actually done here. I'm not sure if that was always true on every watch marked "Swiss".
On some of the early 2nd generation (1959-67) Constellations, Omega omitted the "Officially Certified" on the dials. I don't believe that it makes any difference, the movements were properly marked as being adjusted for positions and temperature. At most it may mean that the movements were not send to the COSC for certification, but I think they were anyway. At some point Omega realized that people wanted to see this language on the dials and replaced it.
Hope this helps,
gatorcpa