US Dog Tag : Chains
There have been several different types of chains used
during the evolution of the U.S. dog tag, every thing from shoe laces  
to the current ball chain,  here are some examples.

simple rope
 1943 issue cord

Early 1940's monofilament plastic with metal
ends, yellowed with age.

The above and next two examples solved the major problem
with soft cord, metal chains will withstand fire, above is a
reprint of a quartermaster manual showing the first of
the metal chain set with what are referred to as sister hooks
small clasps on the end of the chains.

This silver set was favored by officers