XmInternAtom(3) on Linux
The text 13d64f2503a6, as Debian 13 (Trixie), Debian 12 (Bookworm), Debian 11 (Bullseye), Debian 10 (Buster), Debian 9 (Stretch), Debian 8 (Jessie) shipped it.
A macro that returns an atom for a given name "XmInternAtom" "atoms"