The fourth digit has been used many times as a way to different later models from earlier ones. As you can see, the exact meaning of the fourth digit depends largely on the preceding digits. Finally, the fourth digit is the finish or stock of your weapon: “1” is nickel or full blue finish, “2” is nickel or blue casehardened finish, “3” is nickel finish, “4” is Royal Blue casehardened finish and “6” is nickel finish.In the third digit is the caliber of the gun: “3” is 3 or 4 inches, “4” is 4.75 or 5 inches, “5” is 5.5 inches, “7” is 7.5 inches and “1” is 12 inches.The second digit of the model code is the weapon caliber: “4” is.“7” and “8” are employed occasionally to specify special variants. It is fitted with the early original Colt marked parts such as the original non-chrome lined barrel with and 's' & '12' marking. This was the the third full year of Colt AR 15 production. As you didn't list the third digit in your rifle's serial number, I posted the two years cut-off for you. The lower receiver has the Colt four line markings with the four digit serial number 'SER SP 07386' indicating production at about Fall, 1966. The 1982 start number was listed at SP158201. ”2” uses a frame from Colt Custom Shop while “3” utilizes a non-standard frame. According to the chronology list in 'Black Rifle II', your rifle was made in 1981 or 1982. For the first digit, “1” means your Colt is the first model with a basic frame while “2”, “3”, “4” etc are the later versions.

Colt sp1 serial numbers