Apple Xserve (Early 2008) - Manual (Page 15)

Chapter 1 Installing the Xserve 15
 If you have a square-hole rack, start with the rail flanges inside the posts and expand
the rail between the front and rear posts until the small round guide on each flange
extends through the rail.
2 Slide a spacer, arrow up, over the small round guide at the ends of each rail and install
the mounting screws finger tight.
Note: Don’t tighten the mounting screws until you align the rails in the next step.
Expand the rails
so they fit inside
the rack posts. Position the spacer
on the outside of the
post, and over the
threaded guide on the rail.
Position the spacer so the small
block on the back engages the
square hole in the rack support. Use the screws that came
with the rail to secure the
spacer to the rail.