Belt buckle of a seat belt, especially for a motor vehicle

The safety belt loop is anchored by its lower end (5) on the movable part of the seat longitudinal guide system. At its upper end (6) it carries a belt clasp (7) able to receive a bolt (8) integral with the safety belt. The clasp is installed to slide vertically in a sheath integral with the movable part so as to take two stable positions. These positions are a retracted position corresponding to the locking of the bolt in the clasp when the clasp is positioned under a fictitious reference point (H) equivalent to the articulation of a passenger's hip sat on the seat, and a deployed position corresponding to the unlocking of the bolt from the clasp when the clasp is above the reference point.