Acer AR320 F1 - Manual (Page 129)

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(3) Make certain the proper mounting holes on the rack post are
selected.
(4) Align two washers on each side to the holes on the rack
mount and the front mounting rails. Use two metal screws to
secure the front mounting rails (1) to the rack.
(5) Align two washers on each side to the holes on the rack
mount and the back mounting rails. Use two metal screws to
secure the back mounting rails (2) to the rack.