special stamps, regular definitive stamps having no special markings. The first definitive series came in February 1950, and featured the Gate of Heavenly Peace against a background of clouds. The nine values ranged from $200 to $10,000. The design was modified several times over the next year, and again in 1954, resulting in what philatelists call the "second" through "sixth" issues, each varying in minor ways, such as the layout of the clouds. The postal system found it necessary to surcharge stamps of the previous government, with issues in March and August 1950, and May 1951. In addition, leftover stamps of the Northeastern Provinces were pressed into service in July 1950, and those of East China in December 1950. In the meantime, various commemoratives marked conferences and other events of the young republic. In June 1952 a set of forty stamps depicting physical exercises was issued in conjunction with a radio program; ten exercises were illustrated, each with a block of four, where each stamp shows a different position of arms and legs for the exercise. Cultural Revolution Reduced stamp sales and withdrawals from issue of several stamps during the Cultural Revolution resulted in a few stamps that are quite scarce, especially used. The typical pattern resulting in rarity was unauthorized sales before the official date of issue by isolated post offices of stamps which were then withdrawn from issue before the official date of issue. One rarity, an unissued 8f stamp from 1967 commemorating the 40th Anniversary of Establishment of Jing Gangshan Revolutionary Base, popularly known as "Big Blue Sky", which pictured Chairman Mao and Lin Biao on the podium overlooking Tienanmen Square exists only as scraps salvaged from the destruction process. Only scraps from one stamp in the withdrawn set are known to exist. During the Cultural Revolution era, between the year of 1967 and 1971, there were 19 sets total 80 stamps issued by the China Postal Service, with its distinct design and strong political overtone, these stamps became highly collectable after the end of Cultural Revolution. Postal service The postal service was established rapidly in the 1950s and 1960s. By 1952 the principal postal networks centered on the capital, Beijing, and links to all large cities had been established. Great progress was made in improving the postal service under the First Five-Year Plan. Postal service was also developed in the rural areas. Besides extending rural postal