This is a great opportunity to own a piece of history.
We have spent months reaching out to colt historians trying to track down history on this specific complete upper. A good majority of the information on the key details this complete upper has, can be found in the Vickers Guide – AR15 Volume 1.
From the information we have gathered it is Pre 1969 Prototype Colt A1 Upper Receiver with a Retro Fitted Transitional style SP1 Colt “C” Bolt Carrier Group. Colt -C MP C- Marked 5.56 Original Barrel that has a 1:12 Twist which was only produced for a limited time as part of a retrofitting program. On the barrel is a hand engraved serial number which represents a rack # only used in Puerto Rico. From what limited knowledge we have received, the only A1 M16 Uppers that went to Puerto Rico were Vietnam Bring Backs. The Gas tube is the easily identifiable Lower bend style used on early A1 Uppers. This upper has original triangle Colt handguard sections with a gray flat slip ring. As far as any historian or ourselves can tell, this is a 100% original complete upper from Colt. There is a distinct rectangle imprint stamp on the left front of the upper receiver as well as the upper not having any C ( Colt ) markings. The only other publicly documented M16A1 to share BOTH of these two very unique features is the North Vietnamese Captured M16A1 that is on display in a museum in Moscow, Russia.