/usr/share/mate-panel/ui/showdesktop-menu.xml is in mate-panel 1.20.1-3ubuntu1.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
1 2 | <menuitem name="Show Desktop Help Item" action="ShowDesktopHelp"/>
<menuitem name="Show Desktop About Item" action="ShowDesktopAbout"/>
|