Nice watch with a ‘swing ring’ case. Do you have pictures of the caliber? it could be either a 12 or 13 lignes, a 23.7 or a rarer 27.9, all of them were cased in this model. Serial number of the caliber dates it between 1915 and 1923, approximatively around 1917-18.
Would be able to say more with a picture of the movement.
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