My '69 Snorkel (ladies version) has a JBC bracelet with tuning fork clasp, basically the exact same style as you have pictured, though obviously smaller
. Given that I've found vintage ladies watches to not be subject to the same degree of modifications as men's watches of the same era, I have no reason to think it's a later addition.
This doesn't answer the "how early" part of your question, however.
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