The Manfrotto 520BALL 75mm Half Ball is designed to facilitate quick leveling of a 75mm ball head with the 3283 tripod, and has a 3/8"-16 threaded screw. It can also be used with the Gitzo G-1422 75mm Bowl Interface Adapter to provide an economical and low profile levelling base with any of the Gitzo Series 3 "systematic" tripods(which feature an interchangeable top plate that accepts rapid or geared center columns as well as levelling bases).