In 1936 Gallet purchased the rights to the waterproof wristwatch case produced by Schmitz Frères & Cie. By modifying the case with sealed gasket pushers, Gallet was able to produce what is widely accepted as the first waterproof wrist chronograph.
With WWII underway, there was a high demand for waterproof chronographs. While some were sold for personal use, many were used for military applications. The caseback of this watch is engraved “W. Sargis USMC 4-1-42”
By searching through US Military databases, I came across a US Marine named William Sargis, who was in the Third Marine Division. The 3rd Division was officially activated in September 1942, when it was sent to New Zealand for training. In 1943, the 3rd fought in the Battle of Bougainville, and in 1944 it fought in the Battle of Guam. Its logical to hypothesize that this watch was purchased in the US by Sargis prior to him heading off to battle in 1942.

3rd Marine Division in Battle of Bougainville

3rd Marine Division in the Battle of Guam
Military records show that William Sargis was wounded in action (WIA) during the Battle of Guam.
A newspaper search turned up a June 13, 1945 wedding announcement in Northampton, Massachusetts, where a US Marine Corporal by the name of Louis Sicard was married. William Sargis was his best-man.
So, it appears Mr. Sargis made it home alive.
Thank you for your service, Mr. Sargis.
I will be enjoying your watch.