HP 354556-002 - Manual (Page 18)

18 HP BladeSystem p-Class Enclosure Installation Guide
NOTE: The depth of an HP or a Compaq branded rack (29 in) is clearly
indicated on the rack rails.
IMPORTANT: Numbers on the rack rail provide a gross adjustment of
the depth of the rack rail. You may need to adjust the rack rail depth
again to ensure a proper fit.
2. Install the right rack rail into the rack:
NOTE: Identify the left (L) and right (R) rack rails by markings stamped
into the sheet metal.
a. Insert the rear of the right rack rail into the rack at the marks you made
when measuring with the template.