/usr/share/kde4/config/workspaceoptionsrc is in plasma-active-default-settings 1:0.4-0ubuntu1.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
1 2 3 | [TitleBarButtons]
DesktopLeft=MS
DesktopRight=HIA__X
|