The first wristwatch was made by Patek, specifically for Hungarian Countess Koscowicz in 1868. Girard Perregaux made large numbers of men's wristwatches in 1880's for the German Military. Men were slow to accept the wristlet or bracelet watch, it was seen as effeminate. The soldiers were the ones who broke this stereotype, and much earlier than I imagined.
This would indicate that wristwatches were indeed in use for decades prior to the production of this particular example. The most common wristwatch would have been one designed for women, and it is possible the subject watch here is just that.
WWI conversions, or trench watches, were done to fill the need for the soldiers who readily realised the advantages of having the time on your wrist.
This is a very interesting watch, I'm still digging.