The GG-W-113 manual winding watch - like the 1974 USMIL-issued Benrus GG-W-113 mechanical here - has both 12- and 24-hour markers, with a very Cold War-esque vibe.  No fancy buttons or bezels.  Just a very simple and reliable watch that does what it is specified to do – tell time on the battlefield.
 
Almost as long as watches have existed, governments and military's have been buying and issuing timepieces to be used in nearly every military scenario imaginable.  Strictly utilitarian, these watches have served as functional instruments, designed to give their wearers specific and accurate information in environments not often encountered by civilian watches.
 
The U.S. Department of Defense association with the issuance of military watches began as DoD specification MIL-W-3818B (usually issued to infantry forces), issued by the DoD for a general-purpose watch for military personal - shortly thereafter, the first contract was awarded in February 1964, making this example here one of the first made.  
 
The DoD specification called for a 17-jewel, hacking wristwatch with an extended service life; however, the 3818B wasn’t around long, eventually morphing into the GG-W-113 here in June 1967.  Of note, the GG-W-113 was uniquely issued to the American Air Force (USAF), beginning with the Viet Nam War, to include fixed wing aircraft and helicopter pilots.
 
To understand military-issued watches, you first need to understand a bit about government purchasing.  When the U.S. government contracts for goods, it does not shop like the rest of us.  Instead of surveying the market and choosing from what is available, it publishes detailed criteria for exactly what it needs. 
 
Private companies - usually American ones, such as Benrus, Belforte, Westclox, Hamilton, Timex, and Stocker and Yale, among others - answer the call, and submit proposals for how they will meet the DoD spec requirements, and at what cost.  As a result, a number of different companies produced U.S. military issue watches under different contracts at different times.  
 
In 1967, the U.S. Department of Defense (DoD) released the GG-W-113 specification, which was issued concurrently for decades thereafter.   Differentiating it from other DoD specs was the 17-jewel hacking mechanical movement and a “sterile” dial for legibility; it was manufactured by Hamilton, Benrus, Marathon, and Altus. 
 
The GG-W-113 was produced from 1967 through 1987, until quartz movements gained in popularity replacing mechanical models like the Benrus here.  Although this model was utilized most prominently during the Viet Nam conflict, there is limited evidence it was also deployed in conflicts within the Persian Gulf and other Near East military theatres during the 1990s.
 
The USMIL specification applied to watch contracts across the federal government, including the armed services.  All will bear case back markings with contract type, federal stock number, manufacturing part number, contract number, manufacture month and year, and serial number - engraved on the caseback of this example:
 
Wrist Watch
GG-W-113
Fed. Stock No. 6645-066-4279 U.S.
MFG. Part No. 39986
Cont. No. GS-06S-9229
Serial No. 37068
Date Dec 1974
 
This Benrus comes on what apepars to be its original-issued nylon ZULU strap, and with leather-backed canvas strap, rugged travel case, and springbar tool.