The lack of automatic text suggests it may be model 2761 or similar. That model used the 420 manual wind movement and were sold in the mid 1950s, around 1954 to 1958. Cases like this were also made locally, by Dennison in the UK for instance. I have one very like it:
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Oh the lack of any markings on the back is also suggestive that it is a Dennison cased model, the Swiss versions had Seamaster and Waterproof text, the UK ones did not.
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