According to this site,http://www.texasphilatelic.org/resources/oldrates.htm air mail (1st ounce) was not 13c until March 2, 1974. The air mail rate was 11c from May 16, 1971 to March 1, 1974, and 10c from January 7, 1968 to May 15, 1971. Since this en...
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