Numerous rifles were rebuilt, Primarily Vietnam era rifles. Typically the barrels were replaced. Some examples of markings uncovered on replacment barrels incorporate: Regrettably, I'm on vacation so I am unable to post images with the bolt. It is MPC engraved. The only marking over the provider would be the "C" https://coltcombatunitrailgun4578888.blogozz.com/30630511/the-basic-principles-of-colt-combat-unit-rail-gun-45