Omega used the "OM" from the late 1960's to the early 1970's. Probably to distinguish dials that were solid gold from those that were just gold colored. These solid gold dials are only found on the higher end Omegas like the Constellation.
Are the very rare? That's a relative term. There certainly weren't a lot of them made, since they were quite expensive at the time and very few could afford these "Deluxe" models. I'm sure some have been melted for gold content over the years.
I suspect that this is one of those items that could be very rare, but that rarity doesn't bring a huge premium given the intrinsic value in the watch from the gold.
gatorcpa
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