Dell N4000 Serie Guide De Mise En Route page 18

Two-post Flush-mount Configuration
1 For this configuration, the castings must be removed from the front side of
each ReadyRails assembly. See Figure 4, item 1 on page 16. Use a Torx™
driver to remove the two screws from each front flange ear (on the switch
side of the rail) and remove each casting. Retain the castings for future
rack requirements. It is not necessary to remove the rear flange castings.
Figure 4. Two-post Flush-mount Configuration
2 Attach one rail to the front post flange with two user-supplied screws.
See Figure 4, item 2.
16
Installation