Dell N2128PX-ON Getting Started Manual page 34

Four-post Threaded Configuration
1 For this configuration, the flange ear castings must be removed from each
end of the ReadyRail assemblies. Use a Torx driver to remove the two
screws from each flange ear and remove each casting. See Figure 1-6, item
1. Retain the castings for future rack requirements.
2 For each rail, attach the front and rear flanges to the post flanges with two
user-supplied screws at each end. See Figure 1-6, item 2.
Figure 1-6. Four-post Threaded Configuration
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Getting Started Guide