Qianlong-marked flasks of this form are extremely rare. Compare the similar vase from the Robert Chang Collection sold at Christie's Hong Kong, 28 November 2006, lot 1319. See, also, another example sold at Christie's Hong Kong, 3 December 2008, lot 2622.
The form of this moonflask was clearly inspired by Yongle blue and white examples, which themselves were based on thirteenth-century Islamic brass prototypes. Examples of the early fifteenth-century porcelain flask include four in the Palace Museum, Beijing, illustrated in The Complete Collection of Treasures of the Palace Museum - 34 - Blue and White Porcelain with Underglaze Red (I), Hong Kong, 2000, nos. 34-37; one in the National Palace Museum, Taiwan, illustrated in Blue-and-White Wares of the Ming Dynasty (I), Hong Kong, 1963, pl. 3; one in the Freer Gallery of Art, Washington D.C., illustrated in Oriental Ceramics, The World's Great Collections, Japan, 1981, pl. 94; and another sold at Sotheby's Hong Kong, 1 November 1999, lot 311.
Compare the Ming and Qing versions of the flask with the Middle Eastern canteen made of brass inlaid with silver, also in the Freer Gallery of Art, illustrated and discussed by John Alexander Pope, 'An Early Ming Porcelain in Muslim Style', Aus der Welt der Islamischen Kunst, Festchrift fur Ernst Kuhnel, (ed. Richard Ettinghausen), Berlin, 1959, pp. 357-75, pls. 1B and 2B, where the porcelain and metal flasks from the Freer Gallery are compared. Both prototypes have a flat back and a slightly domed front, although the Yongle examples are not decorated or glazed on the flat side, while the Islamic flask is inlaid on both sides. The loops on the eighteenth-century flask are taken from both prototypes, as the porcelain ones have only a pair on the shoulders, and the metalware example has only the S-shaped handles flanking the neck. The rectangular foot on the Qianlong flask is a Qing dynasty adaptation, as neither of the earlier flasks were designed to stand upright for display purposes. As a functional object, the metal canteen was probably suspended from a horse's saddle, with the flat side designed to lie flat on the saddle or on a piece of luggage.
The trigrams on the central medallion belong to the Eight Trigrams or ba gua, traditional symbols of divination which were recorded in the Yi Jing, Book of Changes. In this instance, the trigrams Qian and Kun, on opposite sides of the flask, symbolize 'Heaven' and 'Earth', respectively, representing imperial power.