I picked up a used 14.5 inch M4 profile barrel and it has a marking I haven't seen on a Colt barrel before. Where you would expect to find the date code there is just a 20 stamped there. Any ideas as to what this stamp means? The rest of the markings are expected as you can see in the pictures. There is also an "O" stamped on the bottom side of the chamber, which I believe is the mark of the chrome lining vendor. Thanks in advance.