The mag launch button experienced a horizontal line sample on it, as opposed to the circular line sample discovered on the current types, see the image below for an illustration: The "C" on the barrel would point out a chrome chamber only barrel. But AFAIK, Colt marked the "C" up https://cruzbdbzw.creacionblog.com/31089897/indicators-on-colt-m16a1-for-sale-you-should-know