A US Postal Service first day of issue envelope with the "Poland's Millenium" 5 cent stamp, cancelled on Jul 30, 1966. The envelope measures 6.5" x 3.75" and weighs less than 0.1 ounce. The envelope has not been used.
A card measuring 6.5" x 3.5" is included which provides historical background on the Polish Millenium.