Brinker and Lutz, Chinese Cloisonne: The Pierre Uldry Collection, p.74, discuss the significance of the fifth character which occasionally, as in the present example, makes an appearance below a Qianlong four-character mark. Its exact significance is obscure. Sometimes it is simply a number between one and five. Alternatively it can define the use and category of the vessel and in addition it probably carries a reverential and auspicious meaning.