NOS means we bought it from GM: it is a new, unused part and GM no longer makes this part. We may still have stock because we actually have a warehouse stocked with parts. It does not mean that this part will exactly match your old part and be totally 100% identical.
Feel free to call (203) 366-1332 to request a part comparison. Please have the Dr. Rebuild part number handy, so that we may get the part from the warehouse and not tie up your valuable time.