I have one of those....mine was made in 2000 which is the last year for the MT6551
production. It features the A2 style (government profile) 20" barrel.
The prefix of CST = Colt Sporter Target. In the pre-ban days this model was called the Sporter Target (R6551). In the post-ban era it's the Match Target - Target Model (MT6551).
The barrel is 1 pound lighter than the 20" HBAR and about the same weight as the 16" HBAR.