Well, I seem to be "photo challenged" so I'll try to explain.
On the open-top, the timing mark is the left hammer slot cut in the recoil shield. Using that as a "rear sight" as if sighting down the barrel, the locking notch should be just to the left of the sight line when you hear the bolt drop. Lockup will be just to the right of the sight line. ( this is for a six shot revolver) Well dang, the pic showed up (sideways). This is bolt drop position ( notch to the left).
Mike