Abstract: This detector of the type comprising a supporting structure (1) having a seat (2) for the ball (3) and a locking lever (4) articulated (at 5) on the supporting structure about a rotation axle and movable by means of the ball between a retracted position and a position for locking the reeling mechanism is characterized in that the supporting structure (1) possesses two arms (8), at the end of which are formed notches (9) for receiving projecting parts of the lever and constituting the rotation axle of the latter, the lever having a hook (12) designed to interact with a stop surface (13) of the supporting structure in order to keep the projecting parts of the lever in position in the notches.