I can't see a Portuguese customs import stamp on any of the lugs so I would deduce that it's a locally produced case.
Often at that time, movements were imported and cased in their home country as a way of reducing import taxes, especially for precious metals.
I have a very similar Genève presented to my Dad for his retirement, also in 1977.
As for identification, it's an Omega Genève with a locally produced gold case.
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