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Most of the Vintage Omega models are dating by their movement serial numbers, and not by their cases. In curtain models, the inside of the case back might by date stamped with the last 2 numbers of the year (e.g. 67 = 1967) which can be used as a rough guide, although it is often advised to counter check this information with what the movement serial number is telling you.
This is because discrepancies are often observed between caseback dates and movement serial numbers with an accepted 1 year if not 2 years in some cases. The reason for this is simply, the production of most watch parts at the time was not done in accordance to set serial range, but rather what was produced but what was available, perhaps from a later or earlier batch.