Lots of rifles have been rebuilt, In particular Vietnam era rifles. Ordinarily the barrels were replaced. Some examples of markings identified on replacment barrels include: If it is a bell shape, that will be appropriate for your Colt. As well as gasoline tube color and texture is interesting. See In https://coltcombatunitrailgun4526885.ourcodeblog.com/31401911/top-guidelines-of-colt-combat-unit-rail-gun-45