The Apple Lockable Cable Fastener is a metal clip with a hole meant to function as a security device. To use the fastener, several cables would be bundled in the clip and a padlock would be fed through the holes so the device cables and devices (mouse, keyboard, speakers, etc.) could not be easily removed and stolen.
One illustration on the manual shows an Apple Pro Keyboard, Apple Pro Mouse, and the speakers that shipped with the G4 Cube (2001). Thus, this Lockable Cable Fastener likely shipped with a G4 Cube.