The 3416 / 338 is fitted between tripod and head and is used in combination with tripods that don't have built-in levelling mechanisms. Its three adjustment dials enable the operator to make fine, finger tip adjustments up to +/- 5 degrees to ensure the camera is perfectly level. The dials also have locking rings to ensure stability once the right position is reached. A spirit level is built in for reference. The base has a 3/8" female tripod fitting and a 3/8" male head fitting.
One reason why the Manfrotto 338 is popular is because of its Improves tripod function