I believe it originally came from the Omega Constellation. The particular case introduced in the 1960's I think was the third case type used and hence the letter C (third letter). Coupled to that the case actually was thought to resemble a letter C joined to it's own mirror image.
Subsequently all similar type cases began to be called 'C' case.
As every thread ought to have a picture - I have a Vulcain cricket that I consider to be a 'C' case
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