Acer Altos R720 Series User Manual page 172

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(3) Do the same thing to the other mounting rail.
Caution! To avoid personal injury, care should be taken when
pressing the inner rail release latches and sliding the component
into the rack.
2 Attach the inner rails to both sides of the server.
Align the screw holes of the inner rail to the server screws (A),
then slide rails to the left until the rails lock into place with an
audible click (B).
Appendix A: Acer Altos R720 rack installation guide