The phrase is “drawing the fourth corner”. No early account names a piece of chalk. Later tellings provide one, able to draw lines visible only after a reading has gone too far. Modern replicas are deliberately black.
Archivists suspect the object grew out of the words. The Fourth Corner and the Moon supply an atmosphere in which an ordinary stick can look as though it knows where interpretation ends.
Some readers keep a piece nearby as a private reminder to stop. That modest use needs no invisible line and no claim that the chalk is ancient. Drawing on courts or somebody else’s property is no part of the custom.
The inventory calls it a probable invention. A piece can sit unused beside a reading and do its small work. What it cannot do is turn the existence of the prop into evidence for the history told about it.