This watch looks to be a little bit of this and a little bit of that. It would be interesting to hear what Longines Heritage dept has to say about it Do you find the number 399 on the case somewhere? Or just caseback.
Favre Leuba crown. Case back from a different watch (possibly the entire case from a different watch). Possibly Favre Leuba as well. I think the hands have been replaced at a latter date. Dial and movement are 1940s (mid to late 1940s judging from the serial number). I guess this falls under the franken watch category.