Fortunately American gear is relatively easy to date since all US military clothing and gear has a tag or stamp that contains the contract number. By following the numbering system you can easily figure out the year and sometimes even month your item was made. First find out if you have a DA, DSA, DLA, or SPO Contract Number:
If there is only a manufacturer's stamp - it was made prior to 1953.
DA ( Defense Agency) - from 1953 to 1961
DSA (Defense Supply Agency) - from 1962 to 1977 (This is what collectors of Vietnam War gear look for)
DLA (Defense Logistics Agency) - from 1978 to 1993
1994 to Present - SPO, SPM, & other recent numbers.