Jack,
The center sleeve does not rotate and the two screws are there to prevent that from happening. They are in slots in the tube so the erector lens can slide fore and aft for focusing. To adjust the scope merely loosen one of the locking rings and tighten the other thus moving the center sleeve. To focus (remove parallax) use the standard method of looking through the scope at a target set at the range you intend to shoot and move your head sideways or up and down. If the cross hairs appear to move the scope is not in focus so keep adjusting and head shaking until the cross hairs cease to move. Lock the center sleeve in place and record the settings for future reference.
                                                         Rich