Compare a similarly enameled and slightly larger teapot of the same shape, but bearing the raised mark, Jingxi Ling Wanyu zhi (Made by Ling Wanyu, Jingxi [Yixing]), sold at Sotheby's Hong Kong, 27 October 1992, lot 90.
Teapots of this square shape were thought to have been an innovation of the Kangxi period potter, Hua Fengxiang, whose inspiration of this form was from archaic bronze square-shaped vessels known as fanghu, discussed by K.S. Lo in The Stonewares of Yixing: From the Ming Period to the Present Day, Hong Kong, 1986. Although the shape existed in the Kangxi period, it was not until the Yongzheng period that 'robin's-egg'-enamel was used on Yixing wares. For a teapot with Robin's egg glaze of the square shape and dated late 18th century, see K. S. Lo Collection in the Flagstaff House Museum of Tea Ware, Part2, HK, 1984, p.138, no. 129.