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That's for Armalites that use the quick-takedown feature. ie, you pop out the rear locking pin, and the top receiver swivels away from the lower receiver, hinged by the front pin.
That hole/space allows the plastic receiver tabs to slide past the gearbox without snapping.
Again, you can use a gearbox without those holes... you'll just lose the quick-takedown feature, and the gun will have to be assembled like a metal receiver, where you install the gearbox in to the top receiver, and then build the rest of the gun around that.
If you're going to a metal receiver, then it doesn't even matter which gearbox you use.
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