From the information we have gathered it is Pre 1969 Prototype Colt SP1 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. 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.