XmSeparatorGadget(3) on Linux
The text 9b66d4e3486c, as Debian 13 (Trixie), Debian 12 (Bookworm), Debian 11 (Bullseye), Debian 10 (Buster), Debian 9 (Stretch), Debian 8 (Jessie) shipped it.
The SeparatorGadget widget class "XmSeparatorGadget" "widget class" "SeparatorGadget"