Issued on November 30, 1942, this mint never hinged block of four features the 15 Chinese cents airmail stamp from the Nationalist airmail series, with a red overprint reading “Restricted for Use in Sinkiang” applied to designate provincial usage. The design, printed in gray-green by the Chinese Bureau of Engraving and Printing (PBMF) using gravure, depicts an aircraft flying over the iconic Great Wall, symbolizing modernity and national defense. Produced without watermark and line perforated 14, this overprint issue was part of China’s regional control efforts during the war-torn Republican era. Catalogued as Scott CN-IM-SI C5, Michel CN-IM-SI 145, Yvert CN-IM-SI PA5, Stanley Gibbons CN-IM-SI 187, and CSS CN-IM AM57. The block displays uniform centering, vivid impressions, and full original gum with no faults, representing an excellent specimen of this scarce regional overprint. Estimate "$190 – 240"