I recently aquired some near-new slab side uppers with 20" pencil barrels. The receivers are clearly Colt with correct markings near the ejection port. The barrel markings are new to me. The only markings are a "M" and triangle or "V" on right side of the barrels just under the FSB. On the underside of the barrel betweeen the FSB and three prong FH is the number "12". Looking under the handguards and at the barrel nut, etc. these barrels appear to be the same vintage as the receivers and not removed since original assembly.
Any ideas on the what these barrels may be? Perhaps early vintage 1/12 prior to start of chrome lining?