LONDON.- An important Imperial spinach green jade double-gourd San Xi Tang seal, of the revered Qianlong period (1736-1795), estimated to sell for over £1m, is one of the outstanding items in Bonhams Fine Chinese Art sale on 17th May, in London. The rare double-gourd shaped seal, measuring 8.7cm long, has been authenticated by the leading Chinese academic in this field, Guo Fuxiang, of the Palace Museum, Beijing. San Xi Tang, (the Hall of the Three Rarities), is situated in the Forbidden City in the western side of the Yangxin Dian (Hall of Mental Cultivation). The Qianlong Emperor kept three prized rare scrolls in the building: Wang Xizhis Kuaixue Shiqing Tie (timely clearing after snowfall), Wang Xianzhis Zhongqiu (Mid-Autumn festival), and Wang Xuns Bai Yuan Tie (letter to Boyuan), amongst other important antiquities. The actual size of the San Xi Tang hall in which the seal was kept is