Later on in Kanbun to Higashi Ajia 漢文と東アジア ("Kanbun and East Asia"), while discussing the origin of various marks used in kanbun to indicate word order and so on, Kin Bunkyō 金文京 relates an entertaining story from the Soushen ji 捜神記 about the editorial mark which means "these two characters are to be read in reverse order." (In Japan, the mark is レ, but there were other [earlier] forms, like a backwards S, a pair of opposing apostrophe-like strokes, etc., and some of these were used in China as well.)

So I found the original of the story and, with the help of this paper by Inokuchi Yoshiharu 井口喜晴, prepared this translation:

[Famed 3rd-century mystic] Guan Lu 管輅 arrived at a field and saw that Zhao Yang 顏超's features indicated an early death. Zhao Yang's father begged Guan Lu to extend his son's allotted span.

"Go home and buy wine and venison," Guan Lu said. "On the Day of the Rabbit, under the big mulberry tree south of the harvested barley fields, you will see two men sitting at a go board. Pour the wine and serve the venison. Whenever one of them finishes one cup, pour another, until the wine's all gone. If they ask you anything, only bow your head, and do not speak. Do this and they will surely save you."

Zhao Yang did as he was told, and sure enough found two men there sitting at a go board. He placed venison and wine before them. Engrossed in their game, they drank the wine and ate the venison without acknowledging Zhao Yang himself.

After a time, the man sitting to the north looked up at Zhao Yang. "Why are you here?" he demanded angrily.

Zhao Yang simply bowed his head.

"We have been drinking his wine and eating his venison for some time now," said the man sitting to the south. "Do we not owe him something?"

"The text is already decided," said the man to the north.

"Let me see the page," said the man to the south. Seeing that Zhao Yang's life was to end at the age of 19 (十九), he took a brush and added the mark indicating that two characters are to be reversed. "You are saved," he said to Zhao Yang. "You will live to the age of 90 (九十)."

Zhao Yang bowed and left. Later, Guan Lu explained to him: "I am glad to have helped you in this joyous achievement. The man sitting to the north was the Big Dipper, and the man sitting to the south was the Southern Dipper. The Southern Dipper pours life into all, while the Big Dipper is lord of death. All people start at the Southern Dipper when they receive life and move inexorably north to the Big Dipper. That is why when we pray, we direct it at the Big Dipper."

Always be nice to your editors.

Note: Guan Lu actually has a Wikipedia entry, but its translation of this story is either a poor one or a different text, and the punchline of the story, i.e. the fact that the Southern Dipper adds an editor's mark to 十九 (19) to indicate that it should be 九十 (90), is garbled. The idea seems to be turning 十 into 九 (so, 19 → 99) instead, but even that is very poorly explained. The Wikipedia version also appends some nonsense about What Hath Divination Wrought, interpolates unnecessary weeping, etc. It's like a Guan Lu Smallville.