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Physically there are no differences in the lower. The only thing that changes are the markings on the left side of the receiver. The upper receiver can vary. The M16VN ought to have no forward assist, no case deflector, and A1 sights. An M16A1 receiver would add the forward assist. The M16A2 adds the case deflector and A2 adjustable sights. The base M4 would also use this profile. When you get to the M4A1 the upper turns to a Picatinny rail and may come with a detachable carry handle. Most "M4s" these days are all flat tops to begin with and it's turned into a very generic descriptor to describe most any short-barrelled M16.
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