Vintage Longines from the war

It's a great heirloom. As an insurance valuation, I'd say at least $2k, because you could replace it with a good quality example from a dealer for that amount. If you tried to sell it (which you're not going to do, obviously), you'd probably be lucky to get half that amount because of some condition issues that collectors care about. As you probably know, when it comes to collectible items, condition is everything. For me, it would be something to display or wear, but different people feel differently about value.

More importantly, if you can have the watch opened, and read the serial number from the movement and the other numbers stamped inside the case-back, you can send them to Longines via a form on their website (along with photos), and they will send you the historical information on that watch from their archives free of charge. You want to go to the following link and request an Extract of the Archives (not a Certificate of Authenticity). If you click on the link and follow along, it should be clear. I think you will need to set up an account.

https://www.longines.com/en-us/certificate-of-authenticity