The Gitzo GS3750DQR Panoramic Disc creates an independently-controlled panning axis directly atop your tripod, and being equipped with a quick release it is intended for use with a levelling base such as Gitzo's own GS5121LVL for their Series 3-5 "Systematic" tripods. It has a built-in bubble level, and is sized to perfectly match the crown and top plate of Gitzo's Series 3 tripods.
The open channel design of the incorporated quick release adapter means that large lenses with tripod collars can be adjusted fore/aft relative to the base for ideal weight distribution-affording better control of your gear and less vibration which ruins shots.
GS3750DQR Panoramic Disc with Quick Release
3/8"-16 to 1/4"-20 Reducer Bushing