I used to have a very early production MK1 and it had a barrel marked the same way. Not long after I bought it, I read an article about Combat Masters that said Detonics used Colt barrels that they cut down then added a sleeve to. I may be mistaken but I believe the article said that the first 100 or 1000 guns used these barrels. After that they used their own. I have since sold the gun but I may still have the magazine with the article. If you have any interest in the magazine, email me. Hope this helps.