In 1901, postal uniform shirts were allowed as outerwear in hot weather.
During the heated term, postmasters may permit letter carriers to wear a neat shirt waist or loose-fitting blouse, instead of coat and vest, the same to be made of light-gray chambray, gingham, light-gray cheviot, or other light-gray washable material, to be worn with turn-down collar, dark tie, and neat belt, all to be uniform at each office.
– 1902 Postal Laws and Uniform Regulations
Notice the Fechheimer Ad? They still manufacture postal uniforms! In fact, they are the manufacturer of the postal shirts, trousers, shorts and skirts for us here at Postal Uniforms Direct!
facts provided by: HISTORIAN UNITED STATES POSTAL SERVICE MAY 2002