Compaq TA1000 Reference Manual page 18

Tape Array Installation 2-5
5. For round-hole racks only: Extend the mounting bracket through the
rear vertical rail holes. Be sure that the mounting bracket fingers are
inserted into the corresponding vertical inner rail holes.
1
Figure 2-4. Round-hole rack bracket alignment
IMPORTANT: The mounting bracket must be parallel from front to rear before you secure
it to the rack. The left and right mounting brackets must be parallel.