ColtKid, I have the original OBR 7.62 from years back (the first customer deliver 18", actually). Initially I ordered it with the CTR and riser (there is a review in here somewhere) and then changed to PRS. I vaguely remember an issue with the buffer wt and spring back then but it was quickly sorted out by the LT crew. Given that, I'd take vectorsc's advice and call CS @ LT. They may not assemble it for you but they WILL get you on the right track.
I just pulled mine out of the safe and have the lower sitting here on the table but I see no distinguishing marks to tell me WTF the buffer/spring combo is. Sorry.
BTW: Mine shoots better than me so congrats on the new addition to your arsenal!!