The Gen. 1 and Gen 2 rack-mount ears are set up for a center-mount scenario suitable for 2-post racks. Gen. 3 rack ears mount the unit so that the front panel is roughly even with the mounting surface.
Note that all of these units are fanless and have vents on the top surface. When rack-mounting you should leave 1 U of space above the unit for cooling.
There are rack standoff brackets available from numerous sources that can be used to offset the mounting surface. Here is one example. 1RU Recessed Rack Adapter