WHAT DOES COLT COMBAT UNIT RAIL GUN 45 MEAN?

Been clear of the board for a while as a result of relationship, residence renovations and an old automobile resto, but I'm now acquiring back into firearms and especially the AR15 style. So I assume which makes me an previous semi FNG.    The "C" over the barrel would indicate a chrome chamber only barrel. But AFAIK, Colt marked the "C" up near