wookiee gunship ?
That's where you attach Engine Upgrade or Vectored Thrusters.
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It's not the finest example of models produced for FFG. My problem is with the cheek guns that aren't done too well. There are limits in manufacturing from the translation of the art to a scaled down toy, after all.
What you're talking about is that little circle on the underside, right? It's just a manufacturing artifact. Nothing missing. Here is a pic of the ship from the Rebels show:

