Apple Mac Pro (Mid 2010) - Manual (Page 53)

53 Chapter 3 Making a Good Thing Better
10 Hold the side panel against your Mac Pro and push the latch down to secure the
side panel.
You can attach a lock or cable through a metal loop (under the latch) to secure your
Mac Pro. Flip the metal loop down before you close the latch, then close the latch and
attach your lock or cable through the extended loop.