Abstract: The invention belongs to the field of human life and activities and can be used as an attachment fitting for coupling with various external mobile devices and means installed on boats or other transport means such as vehicles, bicycles, motorbikes, etc. The attachment fitting for coupling with external devices comprising the casing with fasteners in which the axial central opening is executed, circumferentially of which the splines are disposed with the possibility of turning of the spline join through an angle from 15 to 40 degrees, the locking mechanism disposed in the gutter on the side face of the casing with the possibility of the transverse movement and executed in the form of the plate with the shaped opening having the splines on the inner part which are disposed circumferentially, and the shaped segment executed with the deviation from the circle centre on the value of not less than two heights of the spline tooth.