Go to Bushmaster's website: parts & accessories, buttstocks & pistol grips, individual buttstock parts, 5310-917-1153 Telestock 4 position Locking Nut should be the part you are looking for. I'll look and see if I can find my receipt and verify part number. I have one from Essential Arms that came with a CAR stock, and think it looks like crap compared to the one I ordered from BM.
Mac
ETA: Around six months ago when I ordered from Bushmaster the lock ring was the type on the left with the holes for the spanner wrench.