Looks like this might be a custom made case. Post some pictures of the front so we can provide ore information.

 

Dear Yupi,

I´m from Brazil (São Paulo). Long time reader here.
AM refers to the state of Amazonas.
ZF means "Zona Franca" = free trade zone
Manaus is the capital of Amazonas and where the free trade zone is located.
There are many companies that have some commercial benefits when producing in the free trade zone, that was very active in the 1970´s/80´s.
It´s probably a custom made case but without pictures from the dial/movement, it´s hard do give more information.
Hope this helps.
Best regards.

 

Obrigado Wolf,
I'm from Portugal so the story is plausible. Brazilian custom made case is good or bad in relation to the swiss omega made (value, gold and prodution quality).

 

De nada Yupi, you´re welcome.
At that time, due to taxes/fees benefits, many companies used to produce or assemble products in Manaus (it still happens). Omega may have followed this procedure or had an agreement with a local supplier.
This may be a gold case and bracelet (in Brazil it´s usually 18K/750 gold) or gold plated. If you open the caseback there may be more info inside.
I believe that the quality had to follow Omega´s standards for that period of time.
How much is it worth? It´s difficult to say, it´s a woman´s watch, its weight, how desirable it is, etc.
Hope this helps.

Saudações/Regards

 

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