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I've don't know specifically about FBI's flush axle but most flush axles I've seen seem to work the same.
The side opposite the axle nut is part of the axle. It has ribbed segments around the standoff that create a pressed fit into the lower arm bore as the axle nut is tightened. Then a flush cap with an o-ring is tapped into the bore of the lower arm to cover the axle nut. I use blue Loctite on everything.
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