As air mail service rapidly improved after 1929, there was an increase in the sale of air mail stamps. New air mail stamps were issued in a rectangular shape, still with the Great Wall and an airplane as the central design, but the airplane in the new issue is a monoplane instead of the old type biplane.　　The stamps, commonly known as〝三次航空〞(Air Mail Stamps of 3rd Print), consisting of ten denominations per set, were issued in three periods. Those of 15￠, 30￠, 45￠, 60￠and 90￠denominations were issued on August 29, 1932. After the revision of air mail postage in 1933, the 25￠, 50￠and $1 denominations were issued on May 13, 1933. While the other denominations ($2 and $5) were issued on June 16, 1937.