Welcome to the forum and thanks for sharing this beauty!
As said above, get a competent person to take the caseback off and get some pictures of the inside caseback and the movement.
Due to tariffs and restrictions on gold watches, it was common to import movements and to case later and I believe that is what we have.We may not have a specific model number.
However, the movement number will help us date things.
The dial has been repainted at some point and that used to be a common practice when servicing. Watchmakers have some disease forcing them to restore "new and shiny".
The real value of this watch is in viewing the exact image your Grandfather lived with and imagining the travels taken together. How cool to be reminded of him everytime you look at this beauty.
It has some gold value but the fortune is in the history.
Please get back to us when you get some more photo's and we will discuss what needs to be done to make this wearable for the millenia (in dry weather, don't get wet).
Take good care of your future heirs watch!