James Cahill writes in "Chinese Erotic Painting" that;

"The Secret Play on a Spring Night that the late Ming collector Zhang Chou, as related above, took to be a work by the eighth-century master Zhou Fang, was a single group composition in handscroll form. Although this painting itself is presumably long lost--no actual erotic painting datable before the late Ming period is known to me--a fenben (sketch copy) has recently come to light that appears from its style to preserve the outlines at least of a pre-Song work, and from its subject to be based on this very "Secret Play on a Spring Night" (Fig. 6). The record in Zhang Chou’s catalogue attests that such a painting was extant in the early seventeenth century, and for it to have survived into the early or mid-nineteenth, from which time this fenben would appear to date (again, on the basis of style), would not be surprising. Fenben copies were made by artists from old paintings they encountered, for use in their own finished works; an example by Gu Jianlong was introduced in PUP Chap. 1 (Fig. 1.4). The picture transmitted in this fenben not only conforms loosely to the Zhou Fang style as it is known from copies, but also differs from other extant erotic pictures in so many respects as to locate it strongly in a very different, much earlier stage in the development of the genre."^[1]