WHAT DOES COLT COMBAT UNIT RAIL GUN 45 MEAN?

The M16A2 buttstock is about 5/8" of an inch lengthier then the sooner models.  Because of the increased size it used a longer buttstock screw along with a spacer.  The swivel is mounted nearer for the buttplate. This buttstock is currently utilized about the M16A4:I understood After i initial saw it inside the rack it was something 'Distinctive'