Dating hamilton watch models
A watch like this one with the sub second hand at 6 will feature a 17 jewel 987A movement.The case will have a screw-on back cover, with a movement ring and dust cover inside, along with a gasket. Depending on the watch it will have a 17 jewel 987S movement inside, or it could have an 18 jewel 2987 in specific naval aviator watches.it is among the highest quality consumer items ever made in the USA. It's varied a little over the months, but has never been more than ±15 seconds different from WWV.
Military personnel also were issued time pieces when they went "into harms way".
Pocket watches were also produced using the 21 jewel 992B movement as well as the 22 jewel 4992B with a sweep second.
Here's an example of the 22 jewel 4992B that my wife's grandfather used during the war while serving in the Pacific as a Naval Flight Officer (Navigator). This one below is interesting as it's a Railway Special but marked US Army on the movement (on the left side at about ). If you compare the 992B movement and the 4992B movement you'll see they are very similar.
Early cases were engraved on the back with "Ord Dept USA" and a serial number starting with OD - representing a non-waterproof case with 15-17 jewels.
The Navy cases were marked differently on the back.