Looking at a Boston Screw-top from the upper wire groove ridge, it appears that there is a six faced glass nut sitting on a glass donut washer and that there is a screw projecting up through the nut. My observations are that the nut portion of the blue aqua CD 158.9 shown here, is sitting just on top of the donut washer and the screw projects 1-1/8 inches above the nut.