The serious fluctuation of the Gold Yuan notes after April 1949 necessitated such frequent revisions of postal tariff that the new stamps became unusable for use immediately after they came off the press. Consequently, a set of unit stamps for various categories of mail matter was issued, thus rendering them valid for sale at whatever price the new rate of postage might be. 　　The unit air mail stamps used in Western Szechwan were those converted from face-value air mail stamps, then in stock in the Western Szechwan District Head Office, by an overprint bearing No. 5 Sung characters〝國內航空費每重20公分〞(domestic air mail postage per 20 grams) and No. 6 Sung character〝蓉〞(Chengtu) in black in the following manner：(1)Surcharged air mail stamps of Hongkong Print　　The stamps have the character〝蓉〞(Chengtu) overprinted in the middle, the characters〝國內航空費〞(domestic air mail postage) overprinted in two vertical lines in the upper right corner, and the characters〝每重20公分〞(per 20 grams) overprinted also in two vertical lines in the upper left corner. The original face value in the lower two corners was deleted by two cross`like decorations.(2)Surcharged air mail stamps of Shanghai Print　　The character〝蓉〞(Chengtu) was overprinted above the air-plane, while other overprinted characters〝國內航空費〞(domestic air mail postage) and (每重20公分)- (per 20 grams) were arranged in two horizontal lines immediately below the wording〝中華民國航空郵票〞(Air Mail Postage Stamps of the Republic of China). The altered value previously overprinted on the stamps vertically in Chinese characters and harizontally in Arabic figures was also deleted with two parallel lines.