It is, of course, very unusual to find such accuracy on any mechanical wristwatch, let alone that is 40+ years old. But bear in mind that the GP HF chronometers were designed for exceptional accuracy, and won plenty of Observatory awards to confirm the successful design.
For example, they took the first FIVE places at Neuchâtel in 1970 (see ad below), and when sold were guaranteed to accuracy to within
two seconds a day!
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