XmGetTearOffControl(3) on Linux
The text 29a5e8370574, as Debian 13 (Trixie), Debian 12 (Bookworm), Debian 11 (Bullseye), Debian 10 (Buster), Debian 9 (Stretch), Debian 8 (Jessie) shipped it.
A RowColumn function that obtains the widget ID for the tear-off control in a menu "XmGetTearOffControl" "RowColumn functions" "XmGetTearOffControl"