A lot of articles say that its easier to date vintage clothing by checking out the Union Labels attached to them. Depending on the color and design of the label, you can find out when a certain vintage find was made.
Here are some useful bits of info from E-bay…
The trademark for the new label was submitted in 1963 with a first use date of June 28, 1963. It was registered April 21, 1964. The new label has the same scalloped circle with the threaded needle and the Int’l Ladies Garment Workers Union around the edge. The center changed to a dark circle crossed by ILGWU in white. In the dark arc above ILGWU, it says Union Made. In the dark arc below ILGWU, it says AFL-CIO. This label was usually printed in blue on white. Note the (R) on the label indicates it was printed after the TM registration date.
We have two examples of labels in other colors used in the early 1970s. They are both from foundation garments. There have been other examples with orange or red lettering, usually on lingerie or nightwear, and recently a black label was presented that dated from 1988-1995.
In 1974, ILGWU decided to change the colors of its label to red, white and blue. This is recorded in the records of the 35th annual convention, archived at the Kheel Center at Cornell. The Report of the General Executive Board to the 35th convention in Miami Beach, Florida, May 31, 1974 mentions the change in colors to “an all-American red-white-blue and the addition of the line ‘Made in USA'” on page 79. (emphasis added) They explain it in part as a result of the need to support the fight against the competition from low-wage imports, particularly to strengthen the nation-wide anti-imports campaign. All label material prepared in conjunction with the campaign used the slogan: “Buy American: The Job You Save May Be Your Own.”