I've RIA drawings specifing barrel markings on a brand new USGI rifle and that marking isn't authorized. The "C" on the barrel would reveal a chrome chamber only barrel. But AFAIK, Colt marked the "C" up close to the muzzle on their early barrels, not underneath the handguards. Pic of https://marcoamvub.tinyblogging.com/examine-this-report-on-colt-m16a1-for-sale-75081065