M1940 Identification Tags
The United States Army implemented identification tags (dog tags) in December of 1906. The original specifications were for one tag only.
The second tag was added in July of 1916. This tag was intended to be removed upon the death of a soldier and buried with his body for identification purposes. In February of 1918 the Army added serial numbers to the tags.
In 1940 the M1940 identification tag was introduced, and four variations of this tag were produced during World War II. Soldiers were required to wear their identification tags whenever practical.
Notes
The most obvious appearance of dog tags in the film is after the death of Private Caparzo, when Captain Miller pulls Caparzo's second tag, and in the scene in the glider field where members of the squad sort through a sack of dog tags from dead paratroopers.